Differential expression of antenna and core genes in Prochlorococcus PCC 9511 (Oxyphotobacteria) grown under a modulated light-dark cycle

The continuous changes in incident solar light occurring during the day obl ige oxyphototrophs, such as the marine prokaryote Prochlorococcus, to modul ate the synthesis and degradation rates of their photosynthetic components finely. How this natural phenomenon influences the diel expression of photo synthetic genes has never been studied in this ecologically important oxyph otobacterium. Here, the high light-adapted strain Prochlorococcus sp. PCC 9 511 was grown in large-volume continuous culture under a modulated 12 h-12 h light-dark cycle mimicking the conditions found in the upper layer of equ atorial oceans. The pcbA gene encoding the major light-harvesting complex s howed strong diel variations in transcript levels with two maxima, one befo re the onset of illumination and the other near the end of the photoperiod. In contrast, the mRNA level of psbA (encoding the reaction centre II subun it D1), the monocistronic transcript of psbD (encoding D2) and the dicistro nic transcript of psbDC were all tightly correlated with light irradiance, with a minimum at night and a maximum at noon. The occurrence of a second p eak during the dark period for the monocistronic transcript of psbC (encodi ng one of the PS II core Chi a antenna proteins) suggested the involvement of post-transcriptional regulation. Differential expression of the external antenna and cove genes may constitute a mechanism of regulation of the ant enna size to cope with the excess photon fluxes that Prochlorococcus cells experience in the upper layer of oceans around midday. The 5' ends of all t ranscripts were mapped, and a conserved motif, 5'-TTGATGA-3', was identifie d within the putative psbA and pcbA promoters.