INACTIVATION OF A SYNECHOCYSTIS SP STRAIN PCC-6803 GENE WITH HOMOLOGYTO CONSERVED CHLOROPLAST OPEN READING FRAME-184 INCREASES THE PHOTOSYSTEM-II-TO-PHOTOSYSTEM-I RATIO

A gene of the unicellular cyanobacterium Synechocystis sp strain PCC 6 803 that is homologous to the conserved chloroplast open reading frame orf184 has been cloned and sequenced. The nucleotide sequence of the gene predicts a protein of 184 amino acids with a calculated molecular mass of 21.5 kD and two membrane-spanning regions. Amino acid sequenc e analysis showed 46 to 37% homology of the cyanobacterial orf184 with tobacco orf184, rice orf185, liverwort orf184, and Euglena gracilis o rf206 sequences. Two orf184-specific mutants of Synechocystis sp PCC 6 803 were constructed by insertion mutagenesis. Cells of mutants showed growth characteristics similar to those of the wild type. Their pigme nt composition was distinctly different from the wild type, as indicat ed by an increase in the phycocyanin-to-chlorophyll ratio. In addition , mutants also had a two- to threefold increase in photosynthetic elec tron transfer rates as well as in photosystem II-to-photosystem I rati o-a phenomenon hitherto not reported for mutants with altered photosyn thetic characteristics. The observed alterations in the orf184-specifi c mutants provide strong evidence for a functional role of the orf184 gene product in photosynthetic processes.