NtcA, LexA and heptamer repeats involved in the multifaceted regulation of DNA repair genes recF, recO and recR in the cyanobacterium Nostoc PCC7120

Abstract

Regulation of DNA repair genes in cyanobacteria is an unexplored field despite some of them exhibit high radio-resistance. With the RecF pathway speculated to be the major double strand break repair pathway in Nostoc sp. strain PCC7120, the regulation of recF, recO and recR genes was investigated. Bioinformatic approach-based identification of promoter and regulatory elements was validated using qRT-PCR analysis, reporter gene and DNA binding assays. Different deletion constructs of the upstream regulatory regions of these genes were analysed in host Nostoc as well as heterologous system Escherichia coli. Studies revealed: (1) Positive regulation of all three genes by NtcA, (2) Negative regulation by LexA, (3) Involvement of contiguous heptamer repeats with/without its yet to be identified interacting partner in regulating (i) binding of NtcA and LexA to recO promoter and its translation, (ii) transcription or translation of recF, (4) Translational regulation of recF and recO through non-canonical and distant S.D. sequence and of recR through a rare initiation codon. Presence of NtcA either precludes binding of LexA to AnLexA-Box or negates its repressive action resulting in higher expression of these genes under nitrogen-fixing conditions in Nostoc. Thus, in Nostoc, expression of recF, recO and recR genes is intricately regulated through multiple regulatory elements/proteins. Contiguous heptamer repeats present across the Nostoc genome in the vicinity of start codon or promoter is likely to have a global regulatory role. This is the first report on the involvement of NtcA and heptamer repeats in regulation of DNA repair genes in any organism.