Differential regulation of ssb genes in the nitrogen‐fixing cyanobacterium, Anabaena sp. strain PCC7120¹

Abstract

Anabaena sp. PCC7120 possesses three genes coding for single-stranded DNA-binding (SSB) protein, of which ssb1 was a single gene, and ssb2 and ssb3 are the first genes of their corresponding operons. Regulation of the truncated ssb genes, ssb1 (alr0088) and ssb2 (alr7559), was unaffected by N-status of growth. They were negatively regulated by the SOS-response regulatory protein LexA, as indicated by the (i) binding of Anabaena LexA to the LexA box of regulatory regions of ssb1 and ssb2, and (ii) decreased expression of the downstream gfp reporter gene in Escherichia coli upon co-expression of LexA. However, the full-length ssb gene, ssb3 (all4779), was regulated by the availability of Fe and combined nitrogen, as indicated by (i) increase in the levels of SSB3 protein on Fe -depletion and decrease under Fe -excess conditions, and (ii) 1.5- to 1.6-fold decrease in activity under nitrogen-fixing conditions compared to nitrogen-supplemented conditions. The requirement of Fe as a co-factor for repression by FurA and the increase in levels of FurA under nitrogen-deficient conditions in Anabaena (Lopez-Gomollon et al. 2007) indicated a possible regulation of ssb3 by FurA. This was substantiated by (i) the binding of FurA to the regulatory region of ssb3, (ii) repression of the expression of the downstream gfp reporter gene in E. coli upon co-expression of FurA, and (iii) negative regulation of ssb3 promoter activity by the upstream AT-rich region in Anabaena. This is the first report on possible role of FurA, an important protein for iron homeostasis, in DNA repair of cyanobacteria.

Functional Dissection of Genes Encoding DNA Polymerases Based on Conditional Mutants in the Heterocyst-Forming Cyanobacterium Anabaena PCC 7120

“…Recent studies showed that in Anabaena PCC 7120, the autocleavage activity of LexA is observed only at alkaline pH and is independent of RecA (Kumar et al, 2015), a mechanism of action different from that of other bacteria. Further experiments from the same group showed that LexA binds to the predicted SOS operator sequence in the promoter region of ssb1 (alr0088) and ssb2 (alr7559), which are part of the SOS regulon (Kirti et al, 2017). This data supports the role of LexA as a repressor of the SOS genes in Anabaena PCC7120.…”

“…Previous studies suggested that a SOS-like response may also exist in Anabaena, but the mechanism may differ as compared with E. coli. The three genes lexA, recA, and ssb1 are among the genes most extensively studied in E. coli and much less in Anabaena (Kirti et al, 2013(Kirti et al, , 2017Kumar et al, 2015). ssb1 was reported to be under the control of LexA and its expression is enhanced following exposure to DNA damaging factors such as mitomycin C (Kirti et al, 2013(Kirti et al, , 2017.…”

Iron Deficiency Generates Oxidative Stress and Activation of the SOS Response in Caulobacter crescentus

“…These results suggest that Fur is involved in their regulation, probably indirectly. In Anabaena sp 7120 FurA acts as repressor of a single-strand DNA binding protein (SSB) that plays a role in recombination and repair by protecting the single-stranded DNA, indicating a possible role of Fur in DNA repair in cyanobacteria (Kirti et al, 2017). …”

Cd-induced cytosolic proteome changes in the cyanobacterium Anabaena sp. PCC7120 are mediated by LexA as one of the regulatory proteins

“…On the other hand, LexA of Anabaena sp. PCC7120 (hereinafter referred to as Anabaena ), exhibited RecA-independent but pH-dependent cleavage (Kumar et al, 2015), and was found to function as a global regulator for genes belonging to several functional categories (Mazón et al, 2004; Sjöholm et al, 2007; Kirti et al, 2017; Kumar et al, 2018; Pandey et al, 2018; Rajaram et al, 2022; Pradhan et al, 2022 PREPRINT; Srivastava et al, 2022a; Srivastava et al, 2022b PREPRINT). The regulation by LexA in Anabaena was through binding to the AnLexA-Box defined as AGT-N 4-11 -ACT with one base mismatch allowed within a 4-base palindrome (Kumar et al, 2018; Srivastava et al, 2022a).…”

“…The regulation by LexA in Anabaena was through binding to the AnLexA-Box defined as AGT-N 4-11 -ACT with one base mismatch allowed within a 4-base palindrome (Kumar et al, 2018; Srivastava et al, 2022a). The binding has been shown for a few DNA repair genes (Kirti et al, 2017; Kumar et al, 2018; Pandey et al, 2018; Pradhan et al, 2022 PREPRINT), oxidative stress-alleviating and C-metabolism genes (Kumar et al, 2018), photosynthetic electron transport chain (pETC) components (Srivastava et al, 2022a), and a few γ-radiation response genes (Srivastava et al, 2022b PREPRINT). The global regulatory role of LexA, both as an activator and a repressor, may be the reason for the non-viable nature of the lexA mutant of Anabaena (Kumar et al, 2018).…”