Sixteen and -associated’ genes (expressed only under conditions of nitrogen fixation) in sp. strain RF-1 have been cloned and sequenced. All of the and -associated genes identified in RF-1 were arranged in a continuous cluster spanning approximately 18 kb and containing seven operons. The operon () has been reported previously. and were found to be located upstream of the operon. were expressed as an operon. A -like gene was found to be located just upstream of and the -associated and were found to be located downstream from The genes located downstream from are arranged and span approximately 7 kb. The function of the ORF situated between and is not known. However, it was identified as a counterpart of ORF-2 in sp. strain PCC 7120 based on the deduced amino acid sequence. Northern hybridization and primer extension analysis indicated that the and -associated genes are organized in and -containing operons, respectively. According to the results of this study and previous reports, the genes are expressed in a rhythmic pattern with peaks during the dark phase when the culture is grown in a 12 h light/12 h dark regimen. The rhythm persisted after the culture was transferred to continuous illumination.


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