Rec(F/O/R) proteins of the nitrogen-fixing cyanobacterium Nostoc PCC7120: In silico and expression analysis

The high radioresistance of Nostoc sp. strain PCC7120 is indicative of a robust DNA repair pathway. In the absence of NHEJ pathway and the canonical RecBCD proteins, the RecF pathway proteins are expected to play an important role in double strand break repair in this organism. The RecF, RecO and RecR proteins which are central to the RecF pathway have not been characterised in the ancient cyanobacteria, several of which are known to be radioresistant. The characterisation of these proteins was initiated through a mix of in silico, expression and complementation analysis. Differential expression of the recF, recO and recR genes was observed both at the transcript and the protein level under normal growth condition, which did not change significantly upon exposure to DNA damage stresses. Expression of RecR as a 23 kDa protein in vivo in Nostoc PCC7120 confirmed the re-annotation of the initiation codon of the gene (alr4977) to a rare initiation codon 'GTT' 267 bases upstream of the annotated initiation codon. Of the three proteins, Nostoc RecO and RecR proteins could complement the corresponding mutations in Escherichia coli, but not RecF. The Nostoc RecO protein exhibited low sequence and structural homology with other bacterial RecO protein, and was predicted to have a longer loop region. Phylogenetic as well as sequence analysis revealed high conservation among bacterial RecR proteins and least for RecO. In silico analysis revealed a comparatively smaller interactome for the Nostoc RecF, RecO and RecR proteins compared to other bacteria, with RecO predicted to interact with both RecF and RecR. The information gathered can form a stepping stone to further characterise these proteins in terms of deciphering their interactome, biochemical and physiological activities. This would help in establishing their importance in RecF pathway of DSB repair in Nostoc PCC7120.