PHOTOSYSTEM-II GENES ISIA, PSBDI AND PSBC IN ANABAENA SP PCC-7120 - CLONING, SEQUENCING AND THE TRANSCRIPTIONAL REGULATION IN IRON-STRESSEDAND IRON-REPLETED CELLS

Under conditions of iron deprivation cyanobacteria produce flavodoxin to replace ferredoxin as the terminal electron acceptor of photosynthe sis. In unicellular cyanobacteria, the gene for flavodoxin is the seco nd open reading frame in a dicistronic operon whose transcription is t ightly regulated by iron. The first gene, isiA, produces a protein tha t is very similar to CP43, a chlorophyll-binding, antenna protein of t he photosystem II reaction center. In the filamentous, heterocystous c yanobacterium Anabaena sp. PCC 7120, isiA and the gene for flavodoxin are located in separate operons with independent promoters. In this pa per, we report on the sequence of isiA and show that it is found in a monocistronic operon that is transriptionally regulated to be expresse d under iron stress but does not produce detectable transcripts under conditions of iron repletion. We also report on the sequence, organiza tion and expression of the gene that codes for CP43, psbC. In Anabaena sp. PCC 7120, psbC has a genetic organization similar to that of othe r cyanobacteria and higher plants; the 5' end of psbC overlaps the 3' end of psbDI. Transcriptional analysis of the psbDC operon showed that it is constitutively expressed in both iron-repleted and iron-stresse d conditions; however, a new monocistronic transcript was detected tha t contains psbC and is preferentially expressed under iron stress cond itions.