Alternative oxygen-dependent and oxygen-independent ribonucleotide reductases in Streptomyces:: cross-regulation and physiological role in response to oxygen limitation

Ribonucleotide reductases (RNRs) catalyse the conversion of ribonucleotides to deoxyribonucleotides and are essential for de novo DNA synthesis and repair. Streptomyces spp. contain genes coding for two RNRs. We show here that the Streptomyces coelicolor M145 nrdAB genes encoding an oxygen-dependent class I RNR are co-transcribed with nrdS, which encodes an AraC-like regulatory protein. Likewise, the class II oxygen-independent RNR nrdJ gene forms an operon with a likely regulatory gene, nrdR, which encodes a protein possessing an ATP-cone domain like those present in the allosteric activity site of many class Ia RNRs. Deletions in nrdB and nrdJ had no discernible effect on growth individually, but abolition of both RNR systems, using hydroxyurea to inactivate the class Ia RNR (NrdAB) in the nrdJ deletion mutant, was lethal, establishing that S. coelicolor possesses just two functional RNR systems. The class II RNR (NrdJ) may function to provide a pool of deoxyribonucleotide precursors for DNA repair during oxygen limitation and/or for immediate growth after restoration of oxygen, as the nrdJ mutant was slower in growth recovery than the nrdB mutant or the parent strain. The class Ia and class II RNR genes show complex regulation. The nrdRJ genes were transcribed some five- to sixfold higher than the nrdABS genes in vegetative growth, but when nrdJ was deleted, nrdABS transcription was upregulated by 13-fold. In a reciprocal experiment, deletion of nrdB had little effect on nrdRJ transcription. Deletion of nrdR caused a dramatic increase in transcription of nrdJ and to a less extent nrdABS, whereas disruption of cobN, a gene required for synthesis of coenzyme B12 a cofactor for the class II RNR, caused similar upregulation of transcription of nrdRJ and nrdABS. In contrast, deletion of nrdS had no detectable effect on transcription of either set of RNR genes. These results establish the existence of control mechanisms that sense and regulate overall RNR gene expression.