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Redox properties of cytochrome b559 (Cyt b559) and cytochrome c550 (Cyt c550) have been studied by using highly stable photosystem II (PSII) core complex preparations from a mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus with a histidine tag on the CP43 protein of PSII. Two different redox potential forms for Cyt b559 are found in these preparations, with a midpoint redox potential ( E'(m)) of +390 mV in about half of the centers and +275 mV in the other half. The high-potential form, whose E'(m)is pH independent, can be converted into the lower potential form by Tris washing, mild heating or alkaline pH incubation. The E'(m) of the low-potential form is significantly higher than that found in other photosynthetic organisms and is not affected by pH. The possibility that the heme of Cyt b559 in T. elongatus is in a more hydrophobic environment is discussed. Cyt c550 has a higher E'(m)when bound to the PSII core (-80 mV at pH 6.0) than after its extraction from the complex (-240 mV at pH 6.0). The E'(m) of Cyt c550 bound to PSII is pH independent, while in the purified state an increase of about 58 mV/pH unit is observed when the pH decreases below pH 9.0. Thus, Cyt c550 seems to have a single protonateable group which influences the redox properties of the heme. From these electrochemical measurements and from EPR controls it is proposed that important changes in the solvent accessibility to the heme and in the acid-base properties of that protonateable group could occur upon the release of Cyt c550 from PSII.

Kinetics of electron transfer from the bound tetraheme cytochrome c to the primary donor (P) have been measured in isolated reaction centers of the purple bacterium Rhodopseudomonas viridis by time-resolved flash absorption spectroscopy. The influence of two major parameters has been studied: temperature (7-305 K) and the redox state of the cytochrome. Most experiments were done with one heme (c-559), two hemes (c-559 and c-556), or three hemes (c-559, c-556, and c-552) poised in a reduced state before the flash. Measurements were done at 1283 nm in the absorption band of P+, and in the region of cytochrome alpha-bands. At room temperature, c-559 donates an electron to P+ with a half-time of 115, 190, or 230 ns (with three, two, or one heme reduced, respectively) and is then eventually rereduced by c-556 (t1/2 = 1.7 microseconds) or by c-552 (in less than 40 ns). The kinetics also include a minor microsecond phase of P+ reduction. At decreasing temperatures, the polyphasic character of P+ rereduction is accentuated. Fast phases (115 ns-10 microseconds) are slightly slowed down, following Arrhenius behavior with a weak activation energy (3.6-8.6 kJ.mol-1), until they become temperature-independent. Their extent decreases rather sharply, at temperatures which vary according to the redox poising: 250, 210, or 80 K when one, two, or three hemes are reduced, respectively. In the last case, P+ can still be reduced at low temperature, apparently directly by c-552 (t1/2 = 1.1 ms, nearly temperature-independent).(ABSTRACT TRUNCATED AT 250 WORDS)

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The rate of charge recombination from the primary quinone to the bacteriochlorophyll dimer of the reaction center from the photosynthetic purple bacterium Rhodobacter sphaeroides has been investigated using time-resolved optical spectroscopy. Measurements were performed at temperatures from 293 to 10 K on reaction centers that have specific mutations that result in a range of 425-780 meV for the free energy difference of charge recombination compared to 520 meV for wild type [Lin, X., Murchison, H. A., Nagarajan, V., Parson, W. W., Allen, J. P., & Williams, J.C. (1994) Proc.Natl.Acad.Sci.U.S.A. 91, 10265-10269]. In all cases, the rate increased as the temperature decreased, although the details of the dependence were different for each mutant. The observed dependence of the rate upon temperature is modeled as arising principally from a several hundred meV change in reorganization energy. The relationships among the rate, temperature, and free energy differences can be well fit by a Marcus surface using two modes centered near 150 and 1600 cm(-1)with a total reorganization energy that decreases from 930 to 650 meV as the temperature decreases from 293 to 10 K. In the inverted region, where the driving force is greater than the reorganization energy, the rate is found to be approximately independent of the free energy difference. This is modeled as due to the additional coupling of high frequency modes to the reaction. An alternative model is also considered in which a 140 meV increase in the reorganization energy is matched by a 140 meV increase in the free energy difference as the temperature decreases. The possible role of solvent dipoles in determining this temperature dependence of the reorganization energy and the implications for other electron transfer reactions are discussed.

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The full chromosomal karyotype of six enological Saccharomyces cerevisiae strains used for fermentation and biological ageing of sherry‐type wines was studied. A genetic method based on the analysis of segregation frequencies of auxotrophic markers, among random spore progeny of hybrids, constructed between laboratory and industrial wine strains (Bakalinsky and Snow, 1990) was used. This method was combined with the analysis of strains by pulsed‐field gel electrophoresis. The results obtained clearly indicate the presence of two, three or four copies of a chromosome in the industrial strains examined, and thus confirm that aneuploidy/polyploidy is not uncommon in these strains. In all strains examined, chromosome XIII polysomy is observed. This chromosome contains the ADH2 and ADH3 loci, that code for the ADHII and ADHIII isoenzymes of alcohol dehydrogenase, which are involved in ethanol oxidative utilization during biological ageing of wines. Tetrad analysis for the ‘flor formation’ character suggests two possibilities: this character is either regulated by at least a digenic system, or by only one gene present on a chromosome which is, at least, disomic.© 1997 John Wiley & Sons, Ltd.