Role of electrostatics in the interaction between plastocyanin
and photosystem I of the cyanobacterium
Phormidium laminosum
Beatrix G. Schlarb-Ridley
1
, Jose
´
A. Navarro
2
, Matthew Spencer
1
, Derek S. Bendall
1
, Manuel Herva
´
s
2
,
Christopher J. Howe
1
and Miguel A. De la Rosa
2
1
Department of Biochemistry and Cambridge Centre for Molecular Recognition, University of Cambridge, UK;
2
Instituto de
Bioquı
´
mica Vegetal y Fotosı
´
ntesis, Centro de Investigaciones Cientı

(2002) Biochemistry 41, 3279–3285]. We discuss the impli-
cations of this conclusion for the divergent evolution of
cyanobacterial and plant plastocyanins.
Keywords: cyanobacteria; electron transfer; photosystem I;
plastocyanin; weak interaction.
Electron-transfer chains like that of oxygenic photosyn-
thesis impose special restraints on the proteins involved.
Reactions must be fast to allow rapid turnover of the
chain. Binding between the reaction partners must be
transient, while at the same time sufficient specificity needs
to be retained. Surface properties of proteinaceous reac-
tion partners play a crucial role in meeting these criteria.
The aim of our research was to increase our understand-
ing of how one property of the protein surface, the charge
pattern, influences the rate constant of the overall reaction
and how it may have evolved. Our model protein is
plastocyanin, a soluble photosynthetic redox protein
which accepts an electron from cytochrome f in the
cytochrome bf complex and passes it on to P
700
+
in
photosystem I. In a previous study [1], we mutated
negatively and positively charged residues on the proposed
interaction site of plastocyanin with cytochrome f and
analysed the reaction of these mutants with the soluble
redox-active domain of cytochrome f (Cyt f) in vitro.This
paper presents results on the interaction in vitro between a
representative subset of these charge mutants with the
physiological electron acceptor of plastocyanin, photosys-

E-mail:
Abbreviations: Cyt f, soluble redox-active domain of cytochrome f;
k
obs
, observed first-order rate constant; k
on
, rate constant of protein
association; k
off
, rate constant of complex dissociation before electron
transfer has taken place; k
et
, rate constant of intracomplex electron
transfer; k
2
, bimolecular rate constant of the overall reaction; k
¥
, k
2
at
infinite ionic strength.
(Received 10 June 2002, revised 5 September 2002,
accepted 15 October 2002)
Eur. J. Biochem. 269, 5893–5902 (2002) Ó FEBS 2002 doi:10.1046/j.1432-1033.2002.03314.x
case in which kinetic data of the interaction of plastocya-
nin with both Cyt f and photosystem I have been
collected in a homologous cyanobacterial system. This is
essential for informed discussion of evolutionary relation-
ships.
The structure and charge properties of plastocyanin have

at 820 nm on reduction by
plastocyanin, and linear dependence of the observed
pseudo-first-order rate constant k
obs
on the plastocyanin
concentration. This type is observed for weak interac-
tions: in a range of experimentally reasonable plastocy-
anin concentrations, no sign of saturation is apparent.
Type II also exhibits monophasic kinetics; however, k
obs
approaches a saturating value at high plastocyanin
concentrations, which provides explicit evidence for
complex formation followed by intracomplex electron
transfer. Type III shows biphasic kinetics, which provides
evidence for the formation of an additional reaction
complex (compared to Type II) so that rearrangement
must occur before intracomplex electron transfer. The
reaction between plastocyanin and photosystem I of
P. laminosum is of Type I [7].
Determination of the ionic strength dependence of rates is
an important method of studying electrostatic interactions
[1]. The salt commonly added to increase ionic strength is
NaCl. However, it has been reported that bivalent cations
can play a specific role in the reaction in vitro between
photosystem I and both plastocyanin [15–17] and cyto-
chrome c
6
[13,18–21] by forming electrostatic bridges
between negative charges on the interacting surfaces. In
this study, we investigated the dependence of the second-

and mutant plastocyanins were carried out essentially as in
Schlarb et al. [23].
Photosystem I preparations
P. laminosum photosystem I particles were obtained by
solubilization with b-dodecyl maltoside as described by
Ro
¨
gner et al. [24] and Herva
´
s et al. [21]. The chlorophyll/
P
700
ratio of the resulting photosystem I preparation was
150 : 1. The P
700
content in photosystem I samples was
calculated from the photoinduced absorbance increase at
820 nm using an absorption coefficient of 6.5 m
M
)1
Æcm
)1
[25]. Chlorophyll concentration was determined by the
method of Arnon [26].
Kinetic analysis
The second-order rate constant, k
2
, and its ionic strength
dependence were measured using laser-flash-induced
absorbance changes of photosystem I at 820 nm. Unless

2
, and correction was made for
the resulting dilution of the reaction mixture. All experi-
ments were carried out at 278, 283, 288, 293 and 298 K.
Thermodynamic activation parameters DH
à
, DS
à
and DG
à
were obtained according to the transition state theory by
fitting plots of k
2
/T vs. T to the Eyring equation:
k
2
T
¼
k
B
h
expðÀDG
z
=RTÞ
¼
k
B
h
expðÀDH
z

exp½ÀV
ii
expðÀ0:3295q
ffiffi
I
p
Þ=ð1 þ0:3295q
ffiffi
I
p
Þ
ð2Þ
where q is the radius of the interactive site (in A
˚
), and the
factor 0.3295
ffiffi
I
p
is the Debye-Hu
¨
ckel parameter j at 298 K
[30]. The allowable error was set to 10
)4
%. For the criteria
used to determine the data range, see the Discussion.
Overall errors in the experimental determination of kinetic
constants were estimated to be 10%.
Electrostatic potentials
Electrostatic potentials of wild-type and mutant plastocy-

[15], and is highly conserved in cyanobacterial plastocya-
nins. Mutagenesis, expression, purification and character-
ization of the plastocyanins has been described [1].
Representations of the electrostatic surfaces showing the
changes introduced by the mutations are displayed in Fig. 1.
The decay of the flash-induced absorbance of P
700
+
at
820 nm was monoexponential for all proteins at each of the
five temperatures (278, 283, 288, 293 and 298 K). In the
range of concentrations and temperatures used in this study,
k
obs
showed no sign of rate saturation. The best interpret-
ation of the results as a whole was a linear response to
plastocyanin concentration through the origin. Examples at
293 K and 298 K are shown in Fig. 2. Thus wild-type and
Fig. 1. Representations of the electrostatic surface potentials of wild-
type and mutant P. laminosum plastocyanin drawn with GRASP [50].
The molecular surface (probe radius 1.4 A
˚
) is coloured according to
electrostatic potential on a scale of red (acidic) to blue (basic). The
orientation is similar to that of Fig. 2 of [1].
Fig. 2. Dependence of k
obs
on plastocyanin concentration: wild-type and
mutant P. laminosum plastocyanin reacting with wild-type P. laminosum
photosystem I at (A) 293 K and (B) 298 K. The data were fitted to the

MgCl
2
show that DG
à
decreases slightly for D44A compared with
wild-type, remains essentially unchanged for K53A, and
increases for all three R93 mutants, most markedly for
R93E (Table 1). Owing to the correlation between DH
à
and
DS
à
, their independent errors, determined by the Exhaustive
Search Method, are large. Hence in all but one case (DH
à
of
R93E), DS
à
and DH
à
lie within the 67% confidence interval
of the wild-type values. However, the trends parallel those
seen for DG
à
: a decrease relative to wild-type for D44A, no
change for K53A, and an increase for all three R93 mutants,
again most pronounced in R93E. It is noteworthy that, with
67% confidence, all DS
à
values are positive under these

lower for mutants of R93.
Response to MgCl
2
. In some systems, enhancement effects
have been reported when bivalent rather than univalent
cations were used in measurements of ionic strength
dependence (see the Introduction). Hence, the dependence
of k
2
of wild-type and all mutants on the concentration of
MgCl
2
was investigated at 278, 283, 288, 293 and 298 K.
Table 1. Kinetic and thermodynamic parameters of the reaction between wild-type and mutant P. laminosum plastocyanin with wild-type P. laminosum
photosystem I. Errors given are either standard errors obtained from curve fitting by least squares (k
2
,k
¥
, DG
à
) or 67% confidence limits derived by
the Exhaustive Search Method (DH
à
, DS
à
).
Plastocyanin
k
2
at 298 K

)1
)
DH
àa
(kJÆmol
)1
)
DS
àa
(JÆmol
)1
ÆK
)1
)
Wild-type 7.1 ± 0.5 10.6 ± 0.5 10.0 ± 0.7 34.06 ± 0.08 40.2 (34.2–46.5) 20.8 (0.3–42.5)
D44A 12.1 ± 0.5 10.9 ± 2.1 11.7 ± 0.2 32.66 ± 0.06 37.9 (34.8–41.1) 17.9 (7.3–28.8)
K53A 7.8 ± 0.1 12.4 ± 0.7 12.3 ± 1.0 33.74 ± 0.08 39.8 (34.5–45.4) 20.7 (2.5–39.8)
R93A 3.3 ± 0.2 5.9 ± 0.5 7.6 ± 1.3 36.00 ± 0.12 47.5 (42.7–52.5) 39.2 (22.9–56.4)
R93Q 4.1 ± 0.1 7.0 ± 0.6 6.7 ± 0.5 35.45 ± 0.11 46.2 (44.4–48.0) 36.6 (30.5–42.9)
R93E 1.3 ± 0.1 8.5 ± 3.4 3.4 ± 0.6 38.37 ± 0.12 50.4 (47.9–52.9) 40.9 (32.4–49.6)
a
Buffer used contained 10 m
M
MgCl
2
.
b
Buffer contained no MgCl
2
; ionic strength was adjusted with NaCl. The first datapoint was not

Nonlinear Eyring plots of the effect of temperature on k
2
at
each salt concentration were used to determine the effect of
ionic strength on the activation enthalpy, entropy and free
energy. No significant difference was observed between the
thermodynamics of the NaCl and MgCl
2
dependencies.
Figure 5A–C shows DH
à
and –TDS
à
at 298 K plotted
against the square root of ionic strength (using MgCl
2
)for
wild-type, K53A and R93E. The noise in the wild-type data
buries any trend, if there is one. Although there is still
considerable noise in the K53A data, a trend in both DH
à
(increasing with ionic strength) and –TDS
à
(decreasing with
increasing ionic strength) is emerging. For R93E, this trend
is clear and considerably larger than any noise. These trends
have also been observed for DH
à
and –TDS
à

NaCl. Eyring plots can be
used to extrapolate k
2
to 318 K [7], the temperature at
which P. laminosum is cultured. When the resulting data
were plotted together with the spinach data at 298 K (an
acceptable growth temperature for spinach), the point of
intersection moved to %150 m
M
NaCl. To our knowledge,
the ionic strength of the thylakoid lumen has not been
determined. Published values of the ionic strength in the
stroma of chloroplasts vary from 130 m
M
to 200 m
M
[34,35], and it seems reasonable to assume that the lumenal
ionic strength lies within a similar range. Hence, at
physiological ion concentrations and temperatures, the
plant and cyanobacterial systems show similar rates.
DISCUSSION
To our knowledge, the work described here and in the
related publications [1,7] is the first kinetic analysis of the
in vitro interactions Cyt f–plastocyanin and plastocyanin–
Fig. 4. Comparison of ionic strength curves obtained by using NaCl or
MgCl
2
: wild-type and mutant P. laminosum plastocyanin reacting with
wild-type P. laminosum photosystem I at 298 K. (A) wild-type; (B)
K53A; (C) R93Q and R93E.

dramatically between the reaction with photosystem I and
with Cyt f. Whereas the reaction with Cyt f showed an
overall attraction between the reaction partners, the reaction
with photosystem I exhibited a repulsion. The attraction
between wild-type plastocyanin and Cyt f could be virtually
abolished by neutralizing a single positive charge (K53A),
whereas the repulsion between wild-type plastocyanin and
Fig. 5. Ionic strength dependence (MgCl
2
)ofDH
à
and –TDS
à
at 298 K.
(A) wild-type; (B) K53A: (C) R93E. Error bars indicate 67% confid-
ence limits obtained by the Exhaustive Search Method [28,29].
Fig. 6. Comparison between P. laminosum and a plant: ionic strength
dependence (NaCl) of k
2
for wild-type P. laminosum plastocyanin
reacting with wild-type P. laminosum photosystem I and wild-type
spinach plastocyanin reacting with wild-type spinach photosystem I.
Experimental data were collected at 298 K; for P. laminosum the rate
constantswerealsoextrapolatedto318K,thetemperatureatwhich
P. laminosum is cultured. The lines represent an interpolation between
the data points.
5898 B. G. Schlarb-Ridley et al.(Eur. J. Biochem. 269) Ó FEBS 2002
photosystem I was abolished by neutralizing a single
negative charge (D44A). The plastocyanin-interaction sites
of Cyt f and photosystem I therefore appear to have a

that changes in position 93 have a more pronounced and
specific effect. For all three R93 mutants, k
¥
is significantly
slower than that of wild-type or the other mutants,
indicating that in addition to the electrostatic effect, which
leads to low rates at low ionic strength, another nonelectro-
static factor, e.g. altered structure of the complex, reduces
the rate at infinite ionic strength.
Response to MgCl
2
. For three mutants, the ionic strength
dependence using MgCl
2
was markedly different from that
using NaCl (Fig. 4B,C). For macromolecular systems,
electrostatic theory, such as the Gouy–Chapman theory
applied to a model membrane [36], can predict stronger
effects for bivalent ions compared with univalent ions at
equivalent ionic strength. However, the fact that not all
plastocyanins in this study show an enhancement effect
suggests a different cause, e.g. binding. A bivalent cation
such as Mg
2+
can function as a bridge between two
negative charges on two interaction sites more effectively
than a univalent cation, and may thus speed up a reaction
by neutralizing repelling acidic groups. Figure 1 shows that,
in comparison with wild-type plastocyanin, where little or
no enhancement effect occurs (Fig. 4A), K53A has gained a

This analysis was based on the transition state theory of
Eyring [37]. The interpretation of the activation parameters
in Table 1 depends on whether the reaction is diffusion-
limited or activation-limited. In the former case, the
transition state would be that of association (k
on
). One
would then expect to see a fast phase with a rate constant
independent of plastocyanin concentration in the experi-
mental traces, which was not observed in this study. Hence
the reaction is likely to be activation controlled, and DG
à
,
DH
à
and DS
à
listed in Table 1 represent more than one
transition state, i.e. that of binding (k
on
and k
off
) and that of
electron transfer (k
et
). With the information to hand, the
magnitude or even sign of each contribution to the measured
parameters cannot be precisely determined. It has to be
remembered that transition state theory was developed for
elementary chemical reaction steps, not for the interaction of

, and it is not
surprising that the measured values for DH
à
were positive
for wild-type and all mutants (Table 1). However, the
situation is different for DS
à
. The loss of translational and
rotational degrees of freedom on complex formation and
the electronic factor make a negative contribution to DS
à
[38]. The only source of positive DS
à
is solvent exclusion
from the complex interface, as water molecules gain degrees
of freedom when leaving the ordered protein solvation shell
and joining the bulk solvent. The fact that DS
à
was positive
for wild-type and all mutants (within at least 67% confid-
ence; Table 1) indicates that solvent effects play an import-
ant role in the interaction between plastocyanin and
photosystem I. When copper proteins react with small
inorganic reagents where little solvent exclusion occurs, DS
à
is negative [40], but when plastocyanin reacts with Cyt f,
involving a relatively large interface [3], it is positive [41]. We
can conclude that the transition state for binding in the
reaction between plastocyanin and photosystem I is parti-
ally desolvated. The importance of desolvation of the

in Table 1 is, by itself, inadequate to explain the direction of
the changes in DH
à
and DS
à
. Most notably, for repelling
reaction partners, an increase in ionic strength would be
expected to decrease the positive DH
à
of binding; any
solvent effect would go in the same direction. The inverse is
thecasefortheattractioninthecaseofAnabaena. Also, if
the structure of the final complex in which electron transfer
takes place remained unaltered with increasing ionic
strength, no significant changes would be expected for the
activation parameters of electron transfer for both P. lami-
nosum and Anabaena. Hence, in both cases, the measured
trend is the opposite of what one would expect.
The observed effect could be explained, however, if one
assumed that increasing ionic strength modifies the structure
of the final complex [17,19]. In the case of P. laminosum,the
final complex may be tighter at high ionic strength, as
repelling electrostatic forces between the reaction partners
have been screened out. In this case, the activation
parameters associated with k
off
and k
et
would experience
additional changes. For k

Modifications in the structure of the final complex may
also influence the transition state of electron transfer. The
most important effects are likely to be on the electronic
factor. If the complex became tighter with increasing ionic
strength, this contribution to DS
à
would become less
negative. The inverse would apply for Anabaena. Hence
for both organisms, the changes in DS
à
of the electronic
factor would have the same sign as the measured trend.
Further discussion of the effects of ionic strength on
thermodynamic parameters can be found in Dı
´
az et al.[19]
and Herva
´
s et al. [14].
Evolutionary implications
Comparison of Cyt f–photosystem I. In the case of the
reaction of plastocyanin with Cyt f, it is clear that the acidic
residues D44 and D45 slow the reaction down and that R93
has a more specific role than K46 or K53. The experiments
reported here were intended to clarify if R93 has the same
specific role in reaction with photosystem I (as is the case for
the analogous residue in Anabaena sp. PCC 7119, R88 [15])
and if the interaction with photosystem I is the reason for
having acidic residues in the interface (especially the better
conserved D44; see alignment in [44]). The results of this

(C. Lange, personal communication).
Comparison of cyanobacterial and plant systems. Figure 6
reveals that, although the reaction between plant plastocy-
anin and photosystem I is faster than that of P. laminosum
at low ionic strength values, the difference disappears in the
region of physiological salt concentrations. This is in
accordance with the results obtained for the reaction
Cyt f–plastocyanin. We have argued previously [1] that
in vitro the plant and cyanobacterial systems reach similar
rates in different ways: the plant system uses mainly
electrostatic interactions, as indicated by the steep decrease
in the rate constant with increasing ionic strength and its
low k
8
. P. laminosum relies on hydrophobic interactions, as
indicated by its shallow ionic strength dependence and the
high k
8
. The importance of hydrophobic interactions has
also been shown in the plastocyanin–photosystem I system
of Synechocystis [14] and Prochlorothrix [46]. It seems
reasonable to conclude from Fig. 6 by extrapolation that k
8
of the plant plastocyanin–photosystem I reaction is lower
than that of P. laminosum. Qualitatively this would lead to
the same conclusion as that drawn for the Cyt f–plastocy-
anin interaction in vitro. The behaviour in vivo, however,
which is of key evolutionary relevance, remains to be
investigated. Studies of both reactions in the green alga
Chlamydomonas reinhardtii have led to the conclusion that,

supported by the Biotechnology and Biological Sciences Research
Council, UK, the Oppenheimer Fund, University of Cambridge, UK,
Corpus Christi College Cambridge, UK, Ministerio de Ciencia y
Tecnologı
´
a, Junta de Andalucı
´
a, Spain and the Research Training
Network ÔTRANSIENTÕ in the Programme Human Potential and
Mobility of Researchers of the European Commission (HPRN-CT-
1999-00095).
REFERENCES
1. Schlarb-Ridley, B.G., Bendall, D.S. & Howe, C.J. (2002) The role
of electrostatics in the interaction between cytochrome f and
plastocyanin of the cyanobacterium Phormidium laminosum. Bio-
chemistry 41, 3279–3285.
2. Kannt, A., Young, S. & Bendall, D.S. (1996) The role of acidic
residues of plastocyanin in its interaction with cytochrome f.
Biochim. Biophys. Acta 1277, 115–126.
3. Ubbink, M., Ejdeba
¨
ck, M., Karlsson, B.G. & Bendall, D.S. (1998)
The structure of the complex of plastocyanin and cytochrome f,
determined by paramagnetic NMR and restrained rigid-body
molecular dynamics. Structure 6, 323–335.
4. Navarro, J.A., Herva
´
s, M. & De la Rosa, M.A. (1997) Co-evo-
lution of cytochrome c
6

¨
er, U., Herrmann, R.G. & Haehnel, W. (1996) The plasto-
cyanin binding domain of photosystem I. EMBO J. 15, 6374–
6384.
11. Xu, Q., Yu, L., Chitnis, V.P. & Chitnis, P.R. (1994) Function
and organization of photosystem I in a cyanobacterial mutant
strain that lacks PsaF and PsaJ subunits. J. Biol. Chem. 269, 3205–
3211.
12. Schubert,W.D.,Klukas,O.,Krauss,N.,Saenger,W.,Fromme,P.
& Witt, H.T. (1997) Photosystem I of Synechococcus elongatus at
4A
˚
resolution: comprehensive structure analysis. J. Mol. Biol. 272,
741–769.
13. Herva
´
s, M., Navarro, J.A., Dı
´
az, A., Bottin, H. & De la Rosa,
M.A. (1995) Laser-flash kinetic analysis of the fast electron
transfer from plastocyanin and cytochrome c
6
to photosystem I.
Experimental evidence on the evolution of the reaction mechan-
ism. Biochemistry 34, 11321–11326.
14. Herva
´
s, M., Navarro, J.A., Dı
´
az, A. & De la Rosa, M.A. (1996) A

mez-
Moreno, C., De la Rosa, M.A. & Tollin, G. (1993) A comparative
laser-flash absorption spectroscopy study of Anabaena PCC 7119
plastocyanin and cytochrome c
6
photooxidation by photosystem I
particles. Eur. J. Biochem. 213, 1133–1138.
19. Dı
´
az, A., Herva
´
s, M., Navarro, J.A., De la Rosa, M.A. & Tollin,
G. (1994) A thermodynamic study by laser-flash photolysis of
plastocyanin and cytochrome c
6
oxidation by photosystem I from
the green alga Monoraphidium braunii. Eur. J. Biochem. 222, 1001–
1007.
20. Herva
´
s, M., De la Rosa, M.A. & Tollin, G. (1992) A comparative
laser flash absorption spectroscopy study of algal plastocyanin
and cytochrome c-552 photooxidation by photosystem I particles
from spinach. Eur. J. Biochem. 203, 115–120.
21. Herva
´
s, M., Ortega, J.M., Navarro, J.A., De la Rosa, M.A. &
Bottin, H. (1994) Laser flash kinetic analysis of Synechocystis PCC
6803 cytochrome c
6

Chem. 21, 316–320.
26. Arnon, D.I. (1949) Copper enzymes in isolated chloroplasts. Plant
Physiol. 24, 1–15.
27. De la Cerda, B., Navarro, J.A., Herva
´
s, M. & De la Rosa, M.A.
(1997) Changes in the reaction mechanism of electron transfer
from plastocyanin to photosystem I in the cyanobacterium
Synechocystis sp. PCC 6803 as induced by site-directed
mutagenesis of the copper protein. Biochemistry 36, 10125–10130.
28. Roelofs, T.A., Lee, C.H. & Holzwarth, A.R. (1992) Global target
analysis of picosecond chlorophyll fluorescence kinetics from pea-
chloroplasts: a new approach to the characterization of the pri-
mary processes in photosystem-II alpha-units and beta-units.
Biophys. J. 61, 1147–1163.
29. Holzwarth, A.R. (1995) Data analysis of time-resolved measure-
ments. In Biophysical Techniques in Photosynthesis (Amesz, J. &
Hoff, A.J., eds), pp. 75–92. Kluwer Academic Publishers, Dord-
recht.
30. Watkins, J.A., Cusanovich, M.A., Meyer, T.E. & Tollin, G. (1994)
A Ôparallel plateÕ electrostatic model for bimolecular rate constants
applied to electron transfer proteins. Protein Sci. 3, 2104–2114.
31. Gilson, M., Sharp, K. & Honig, B. (1988) Calculating electrostatic
interactions in bio-molecules: Method and error assessment.
J. Comput. Chem. 9, 327–335.
32. Sitkoff, D., Sharp, K.A. & Honig, B. (1994) Accurate calculation
of hydration free energies using macroscopic solvent models.
J. Phys. Chem. 98, 1978–1988.
33. Young,S.,Sigfridsson,K.,Olesen,K.&Hansson,O
¨

43. Gabdoulline, R.R. & Wade, R.C. (1999) On the protein–
protein diffusional encounter complex. J. Mol. Recognit. 12,
226–234.
44. Bendall, D.S., Wagner, M.J., Schlarb, B.G., Ubbink, M. & Howe,
C.J. (1999) Electron transfer between cytochrome f and
plastocyanin in Phormidium laminosum.InThe Phototrophic
Prokaryotes (Peschek, G.A., ed.), pp. 315–328. Plenum Press,
New York.
45. Xiao, L. & Honig, B. (1999) Electrostatic contributions to the
stability of hyperthermophilic proteins. J. Mol. Biol. 289, 1435–
1444.
46. Navarro, J.A., Herva
´
s, M., Babu, C.R., Molina-Heredia, F.P.,
Bullerjahn, G.S. & De la Rosa, M.A. (1998) Kinetic mechan-
isms of PSI reduction by plastocyanin and cytochrome c
6
in the
ancient cyanobacteria Pseudoanabaena sp. PCC 6903 and Pro-
chlorothrix hollandica.InPhotosynthesis: Mechanisms and Effects
(Garab, G., ed.), pp. 1605–1608. Kluwer Academic Publishers,
Dordrecht.
47. Soriano, G.M., Ponamarev, M.V., Tae, G.S. & Cramer, W.A.
(1996) Effect of the interdomain basic region of cytochrome f on
its redox reactions in vivo. Biochemistry 35, 14590–14598.
48. Soriano, G.M., Ponamarev, M.V., Piskorowski, R.A. & Cramer,
W.A. (1998) Identification of the basic residues of cytochrome f
responsible for electrostatic docking interactions with plastocya-
nin in vivo: relevance to the electron transfer reaction in vivo.
Biochemistry 37, 15120–15128.