Characterization of two naturally truncated, Ssb-like proteins from the nitrogen-fixing cyanobacterium, Anabaena sp. PCC7120

Abstract

Single-stranded (ss) DNA-binding (Ssb) proteins are vital for all DNA metabolic processes and are characterized by an N-terminal OB-fold followed by P/G-rich spacer region and a C-terminal tail. In the genome of the heterocystous, nitrogen-fixing cyanobacterium, Anabaena sp. strain PCC 7120, two genes alr0088 and alr7579 are annotated as ssb, but the corresponding proteins have only the N-terminal OB-fold and no P/G-rich region or acidic tail, thereby rendering them unable to interact with genome maintenance proteins. Both the proteins were expressed under normal growth conditions in Anabaena PCC7120 and regulated differentially under abiotic stresses which induce DNA damage, indicating that these are functional genes. Constitutive overexpression of Alr0088 in Anabaena enhanced the tolerance to DNA-damaging stresses which caused formation of DNA adducts such as UV and MitomycinC, but significantly decreased the tolerance to γ-irradiation, which causes single- and double-stranded DNA breaks. On the other hand, overexpression of Alr7579 had no significant effect on normal growth or stress tolerance of Anabaena. Thus, of the two truncated Ssb-like proteins, Alr0088 may be involved in protection of ssDNA from damage, but due to the absence of acidic tail, it may not aid in repair of damaged DNA. These two proteins are present across cyanobacterial genera and unique to them. These initial studies pave the way to the understanding of DNA repair in cyanobacteria, which is not very well documented.

Comparative Genomics of DNA Recombination and Repair in Cyanobacteria: Biotechnological Implications

“…In these cyanobacteria (excepted Nostoc ( Anabaena ) PCC7120) one ssb copy is propagated on a plasmid. One of the two Nostoc PCC7120 ssb genes, ( alr0088 , but not alr7579 ) was shown to be involved in the tolerance to UV and mitomycin C which causes formation of DNA adducts (Kirti et al, 2013). …”

The Hypothetical Protein ‘All4779’, and Not the Annotated ‘Alr0088’ and ‘Alr7579’ Proteins, Is the Major Typical Single-Stranded DNA Binding Protein of the Cyanobacterium, Anabaena sp. PCC7120

“…Such correlation was however, not found in post desiccation recovery, (Figure 7B), possibly due to additional stresses, such as osmotic stress experienced during desiccation followed by rehydration of these cells. Of the other two SSB-like proteins of Anabaena , overexpression of Alr0088 decreased the radiation tolerance of Anabaena , while that of Alr7579 had no effect [14]. This suggested that All4779 is the typical bacterial SSB of Anabaena , involved in the repair of single and double strand breaks in DNA, possibly as part of a larger DNA repair complex, which is yet to be identified.…”

“…Survival in response to stress, and post-stress recovery were compared with unstressed/control cultures grown under illumination at 27°C±2°C as described earlier [14].…”

LexA protein of cyanobacterium Anabaena sp. strain PCC7120 exhibits in vitro pH-dependent and RecA-independent autoproteolytic activity

“…This would indicate that the SOS response may not be following the canonical RecA-LexA mediated pathway even in Anabaena, as has been reported earlier for Synechocystis. The only gene with possible role in SOS response studied in Anabaena 7120 is alr0088, which codes for a functional SSB (Kirti et al, 2014), and exhibits a marginal increase in expression in response to UV-B and mitomycin C stresses (Kirti et al, 2013). However, the protein levels of Alr0088 are about 9-10-fold higher than that of LexA in the cell extracts of Anabaena 7120 based on immunodetection assays (Fig.…”

“…The native LexA was tested for its ability to bind DNA using ∼400 bp putative promoter regions of alr0088, which codes for SSB (Kirti et al, 2013;, recA and lexA, all of which have a putative LexA box (supp. Fig.…”