Abstract
The filamentous cyanobacterium Anabaena sp. strain PCC 7120 differentiates specialized cells for nitrogen fixation called heterocysts upon limitation of combined nitrogen in the medium. During heterocyst differentiation, expression of approximately 500 genes is upregulated with spatiotemporal regulation. In the present study, we investigated the functions of sigma factors of RNA polymerase in the regulation of heterocyst differentiation. The transcript levels of sigC, sigE, and sigG were increased during heterocyst differentiation, while expression of sigJ was downregulated. We carried out DNA microarray analysis to identify genes regulated by SigC, SigE, and SigG. It was indicated that SigC regulated the expression of genes involved in heterocyst differentiation and functions. Moreover, genes regulated by SigC partially overlapped with those regulated by SigE, and deficiency of SigC was likely to be compensated by SigE.
Highlights
Heterocysts are terminally differentiated cells of filamentous cyanobacteria that are specialized for the fixation of atmospheric nitrogen
Changes in the expression of sigma factor genes on the chromosome of Anabaena PCC 7120 during heterocyst differentiation were determined by Quantitative Reverse Transcription PCR (qRT-PCR) (Figure 1A)
Transcript levels were increased at 8 h after nitrogen deprivation. These results are consistent with observations using GFP reporter strains, in which the gfp gene on the multi-copy plasmid was expressed from each promoter of sigma factor genes [23], the transcript level of sigE
Summary
Heterocysts are terminally differentiated cells of filamentous cyanobacteria that are specialized for the fixation of atmospheric nitrogen. NtcA and hetR, are necessary for heterocyst differentiation, and their induction by nitrogen deprivation is mutually dependent. NtcA, a transcriptional regulator that globally controls the nitrogen response in cyanobacteria, is activated by binding of 2-oxoglutarate, which is the central signaling molecule reflecting the cellular nitrogen status [7,8]. NtcA activates transcription of the nrrA gene, whose product in turn activates hetR expression [9,10]. Mutation in hetR blocks early steps in differentiation, whereas ectopic expression of hetR induces the formation of heterocysts irrespective of cellular nitrogen status. The hetR gene is required for upregulation of ntcA after nitrogen deprivation, the molecular basis of hetR-dependent regulation of ntcA remains unknown [16,17]
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