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International Symposium on Photosynthetic Prokaryotes Meeting Report
Mobile electron carriers in cyanobacteria
by Chris Nomura (Pennsylvania State U.)

Now that the entire Synechocystis sp. PCC 6803 genome is sequenced, much attention has been given to the study of its mobile electron carriers, especially cytochrome c-M. It has been reported previously that Synechocystis has two main mobile electron carriers: plastocyanin and cytochrome-c6. The main carrier for the organism is plastocyanin when copper is present in the growth media. However, if there is little or no copper available, cytochrome substitutes for plastocyanin as the mobile electron carrier, completing the transfer of electrons from cytochrome b6f to either PS-I or a terminal oxidase.

In a previous study [Zhang et al (1994) J Biol Chem 269:5036] , John Whitmarsh's lab (U. Illinois) reported that even with the deletion of petJ (encoding cytochrome c6) and growth of the organism in copper free media (repressing synthesis of plastocyanin), Synechocystis PCCis still able to grow at near wild type rates. This result suggests that there may be another mobile electron carrier in Synechocystis besides plastocyanin and cytochrome c6. One of the possible candidates to fill this role is cytochrome c-M.

The cytM open reading frame was originally discovered in Synechocystis PCC 6803 by Malakhov et al [(1994) J Plant Physiol 144:259-264]. This initial report indicated that deletion of the gene gives no discernible phenotype when the cells are grown under normal conditions. What about abnormal conditions? John Whitmarsh described the electron transport kinetics of a mutant deficient in cytochrome c6 (by deletion of petJ) and plastocyanin (by deprivation of copper) and of a double mutant deficient in cytochromes c6 and c-M (by mutation of petJ and cytM) and plastocyanin (by deprivation of copper). The results indicate that cytochrome c-M may serve as an electron carrier during photosynthesis. However, with the double mutant, the low levels of plastocyanin would not be high enough to support the degree of electron transfer observed, suggesting that there is yet another mobile electron carrier in Synechocystis.

Michael Malakhov (Sta. Zoologica, Napoli) reported that cytM was expressed only under conditions of stress (temperature and high light) and not during normal growth conditions. Dietmar Pils (U. Vienna) reported that his group could not delete cytM if petE (encoding plastocyanin) was deleted. Several groups reported their inability to create double mutants in petE and petJ under any conditions. This is especially curious since petJ mutants are viable, even when grown in the nominal absence of copper, when the levels of plastocyanin should be very low. It appears that while cytochrome cM may serve as an electron carrier in Synechocystis PCC 6803, it cannot fully substitute for either plastocyanin or cytochrome c6.

The pathways between different species are similar in some ways but quite divergent in others. Oksana Malakhova (Sta. Zoologica, Napoli) reported the cloning of the cytM reading frame from several other cyanobacterial species including Synechocystis PCC 6714, Synechococcus PCC 7942, Anabaena variabilis M3, and Prochlorothrix hollandica. However, cytM could not be detected in Synechococcus PCC 7002.

The role of cytochrome c-M in any cyanobacterium remains to be elucidated. No one has purified the protein product of the cytM reading frame and only Malakhov has detected a transcript. It will be interesting to see the results of future studies of cytochrome c-M and other mobile electron carriers.

(see also Commentary)