The exact identity of cyanobacteria that have been cultured from symbiotic associations with the water spp., whether they are required in the symbiotic process, and their relationship to the symbiotic species, is a matter of some debate. We have characterized a 6 kb region containing the operon and the gene from cyanobacterium 1a, a putative symbiont of . Five complete open reading frames have been sequenced. All are very highly conserved when compared with the corresponding regions of sp. PCC 7120, with 93% to 97% identity at the nucleotide level and 93% to almost 100% at the amino-acid level. The intergenic regions, however, are not highly conserved (53-89% identity) when compared to the corresponding regions of 7120: the genome contains both more copies and more types of short tandemly repeated repetitive sequences than 7120. The startpoints of transcription for both the and operons were mapped and found to be the same as those in 7120. It was not possible to discern an improved consensus promoter sequence, but it was possible to define the likely extent of the promoter to within 40 bases upstream of the transcription start-point.


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