Regulation of Nitrogenase Gene Expression by Transcript Stability in the Cyanobacterium Anabaena variabilis

The nitrogenase gene cluster in cyanobacteria has been thought to comprise multiple operons ; 15 however, in Anabaena variabilis the promoter for the first gene in the cluster, nifB1, appeared 16 to be the primary promoter for the entire nif cluster. The structural genes, encoded by nifHDK1, 17 were the most abundant transcripts; however, their abundance was not controlled by an 18 independent nifH1 promoter, but rather by RNA processing , which produced a very stable nifH1 19 transcript and a moderately stable nifD1 transcript. There was also no separate promoter for 20 nifEN1. In addition to the nifB1 promoter , there were weak promoters inside the nifU1 gene 21 and inside the nifE1 gene and both promoters were heterocyst specific. In an xisA mutant, 22 which effectively separated promoters upstream of an 11 -kb excision element in nifD1 from the 23 downstream genes, the internal nifE1 promoter was functional . Transcription of the nif1 genes 24 downstream of the 11 -kb element, including the most distant genes, hesAB1 and fdXH1, was 25 reduced in the xisA mutant, indicating that the nifB1 promoter contributed to their expression. 26 However, with the exception of nifK1 and nifE1, which had no expression, the other 27 downstream genes showed low -moderate levels of transcription in the xisA mutant. The hesA1 28 gene also had a promoter but the fdxH gene had a processing site just upstream of the gene. 29 The processing of transcript s at sites upstream of nifH1 and fdxH1 correlate d with increased 30 stability of these transcripts resulting in greater amounts compared to transcripts that were not 31 close to processing sites.