An HcpR paralog of Desulfovibrio gigas provides protection against nitrosative stress

Desulfovibrio gigas belongs to the group of sulfate reducing bacteria (SRB). These ubiquitous and metabolically versatile microorganisms are often exposed to reactive nitrogen species (RNS). Nonetheless, the mechanisms and regulatory elements involved in nitrosative stress protection are still poorly understood. The transcription factor HcpR has emerged as a putative regulator of nitrosative stress response among anaerobic bacteria. HcpR is known to orchestrate the expression of the hybrid cluster protein gene, hcp, proposed to be involved in cellular defense against RNS. According to phylogenetic analyses, the occurrence of hcpR paralog genes is a common feature among several Desulfovibrio species. Within the D. gigas genome we have identified two HcpR-related sequences. One of these sequences, hcpR1, was found in the close vicinity of the hcp gene and this finding prompted us to proceed with its functional characterization. We observed that the growth of a D. gigas strain lacking hcpR1 is severely impaired under nitrosative stress. An in silico search revealed several putative targets of HcpR1 that were experimentally validated. The fact that HcpR1 regulates several genes encoding proteins involved in nitrite and nitrate metabolism, together with the sensitive growth phenotype to NO displayed by an hcpR1 mutant strain, strongly supports a relevant role of this factor under nitrosative stress. Moreover, the finding that several Desulfovibrio species possess HcpR paralogs, which have been transmitted vertically in the evolution and diversification of the genus, suggests that these sequences may confer adaptive or survival advantage to these organisms, possibly by increasing their tolerance to nitrosative stress.