TRANSCRIPTIONAL AND TRANSLATIONAL REGULATION OF PHOTOSYSTEM-I AND PHOTOSYSTEM-II GENES IN LIGHT-DARK-GROWN AND CONTINUOUS-LIGHT-GROWN CULTURES OF THE UNICELLULAR CYANOBACTERIUM CYANOTHECE SP. STRAIN ATCC-51142

Cyanothece sp, strain ATCC 51142, a unicellular, diazotrophic cyanobac terium, demonstrated extensive metabolic periodicities of photosynthes is, respiration, and nitrogen fixation when grown under N-2-fixing con ditions, This report describes the relationship of the biosynthesis of photosynthesis genes to changes in the oligomerization state of the p hotosystems. Transcripts of the psbA gene family, encoding the photosy stem II (PSII) reaction center protein D1, accumulated primarily durin g the light period, and net transcription reached a peak between 2 to 6 h in the light in light-dark (LD) growth and between 4 to 10 h in th e subjective light when grown under continuous light (LL). The relativ e amount of the D1 protein (form I versus form 2) appeared to change d uring this diurnal cycle, along with changes in the PSII monomer/dimer ratio, D1 form 1 accumulated at approximately equal levels throughout the 24-h cycle, whereas D1 form 2 accumulated at significantly higher levels at approximately 8 to :LO h in the light or subjective light, The psbD gene, encoding the reaction center protein D2, also demonstra ted differences between the two copies of this gene, with one copy tra nscribed more heavily around 6 to 8 h in the right, Accumulation of th e PSI reaction center proteins PsaA and PsaB was maximal in the dark o r subjective-dark periods, a period during which PSI was primarily in the trimeric form, We conclude that photosystem organization changes d uring the diurnal cycle to favor either noncyclic electron flow, which leads to O-2, evolution and CO2 fixation, or cyclic electron flow, wh ich favors ATP synthesis.