Expression and phylogeny of the multiple antenna genes of the low-light-adapted strain Prochlorococcus marinus SS120 (Oxyphotobacteria)

In contrast to typical cyanobacteria, Prochlorococcus strains possess an in trinsic divinyl-chlorophyll (Chl) a/b-protein complex instead of phycobilis omes as the major light-harvesting system. These pigment-protein complexes are encoded by a variable number of pcb genes depending on the ecotype to w hich the Prochlorococcus strain belongs: low-light-adapted strains possess several pcb gene copies whereas only a single copy is present in high-light -adapted strains. In this study, the light-regulated expression of the seve n pcb genes of Prochlorococcus marinus SS120 was examined. The pcbF gene wa s found to exhibit a high turnover and its mRNA could only be detected as a degraded product under all light conditions. Steady-state levels of transc ripts originating from the six other pcb gene copies varied over several or ders of magnitude but were not significantly differentially regulated by li ght intensity. Transcript levels of most pcb genes increased between 4.5 an d 8.5 mu mol quanta m(-2) s(-1), peaked at 45 mu mol m(-2) s(-1) and decrea sed at the highest irradiance (72 mu mol m(-2) s(-1)). A phylogenetic analy sis of the Pcb proteins and other members of the six-helix Chl protein supe rfamily revealed that PcbC and PcbG make a separate cluster with regard to the other Pcbs from SS120. In contrast, Pcb sequences from four high-light- adapted Prochlorococcus sp. strains were found to cluster together and to b e less variable than SS120 Pcbs. Thus, pcb genes likely evolved at a differ ent rate in the two Prochlorococcus ecotypes. Their early multiplication an d diversification is likely a key factor in the successful adaptation of so me genotypes to very-low-light conditions.