REGULATION, UNIQUE GENE ORGANIZATION, AND UNUSUAL PRIMARY STRUCTURE OF CARBON FIXATION GENES FROM A MARINE PHYCOERYTHRIN-CONTAINING CYANOBACTERIUM

Marine phycoerythrin-containing cyanobacteria are major contributors t o the overall productivity of the oceans. The present study indicates that the structural genes of the carbon assimilatory system are unusua lly arranged and possess a unique primary structure compared to previo usly studied cyanobacteria. Southern blot analyses of Synechococcus sp . strain WH7803 chromosomal DNA digests, using the ribulose 1,5-bispho sphate carboxylase/oxygenase (Rubisco) large subunit gene from Synecho coccus sp. strain PCC6301 as a heterologous probe, revealed the pres e nce of a 6.4 kb HindIII fragment that was detectable at only low strin gency. Three complete open reading frames (ORFs) were detected within this fragment. Two of these ORFs potentially encode the Synechococcus sp. strain WH7803 rbcL and rbcS genes. The third ORE situated immediat ely upstream from rbcL, potentially encodes a homologue of the ccmK ge ne from Synechococcus sp. strain PCC7942. The deduced amino acid seque nces of each of these ORFs are more similar to homologues among the be ta/gamma purple bacteria than to existing cyanobacterial homologues an d phylogenetic analysis of the Rubisco large and small subunit sequenc es confirmed an unexpected relationship to sequences from among the be ta/gamma purple bacteria. This is the first instance in which the poss ibility has been considered that an operon encoding three genes involv ed in carbon fixation may have been laterally transferred from a purpl e bacterium. Analysis of mRNA extracted from cells grown under diel co nditions indicated that rbcL, rbcS and ccmK were regulated at the tran scriptional level; specifically Rubisco transcripts were highest durin g the midday period, decreased at later times during the light period and eventually reached a level where they were all but undetectable du ring the dark period. Primer extension analysis indicated that the ccm K, rbcL and rbcS genes were co-transcribed.