We have cloned and sequenced the (nucleotide reductase) locus of The locus seems to be organized in an operon comprising four ORFs. The first three encode polypeptides highly similar to the product of the coding sequences characterizing the operons of The sequencing of the conditional lethal mutation localized in the cistron, and the lethality of insertional mutations targeted in the internal region of and demonstrated the essential role of this locus. The fourth ORF, part of the putative operon, which is not similar to any known protein, is also essential. The regulation of expression of the operon, monitored by transcriptional fusions, is similar to the regulation of the functionally relevant operon of The operon was induced by thymidine starvation and its expression was directly or indirectly affected by RecA function. Genetic and functional analysis strongly indicates that in the class I ribonucleotide reductase encoded by this operon is evolutionary distant from the homologous class I enzyme of Enterobacteria.


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