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Although cytochrome f from the Antarctic psychrophile, Chlamydomonas raudensis UWO 241, exhibits a lower apparent molecular mass (34 kD) than that of the mesophile C. reinhardtii (41 kD) based on SDS-PAGE, both proteins are comparable in calculated molecular mass and show 79% identity in amino acid sequence. The difference in apparent molecular mass was maintained after expression of petA from both Chlamydomonas species in either E. coli or a C. reinhardtii DeltapetA mutant and after substitution of a unique third cysteine-292 to phenylalanine in the psychrophilic cytochrome f. Moreover, the heme of the psychrophilic form of cytochrome f was less stable upon heating than that of the mesophile. In contrast to C. raudensis, a C. reinhardtii DeltapetA mutant transformed with petA from C. raudensis exhibited the ability to undergo state transitions and a capacity for intersystem electron transport comparable to that of C. reinhardtii wild type. However, the C. reinhardtii petA transformants accumulated lower levels of cytochrome b ( 6 ) /f complexes and exhibited lower light saturated rates of O(2) evolution than C. reinhardtii wild type. We show that the presence of an altered form of cytochrome f in C. raudensis does not account for its inability to undergo state transitions or its impaired capacity for intersystem electron transport as previously suggested. A combined survey of the apparent molecular mass, thermal stability and amino acid sequences of cytochrome f from a broad range of mesophilic species shows unequivocally that the observed differences in cytochrome f structure are not related to psychrophilly. Thus, caution must be exercised in relating differences in amino acid sequence and thermal stability to adaptation to cold environments.