The duplicated hasR and hasL genes of Streptomyces ambofaciens encode alternative sigma factors (named BR and BL) belonging to the B general stress response family in Bacillus subtilis. The duplication appears to be the result of a recent event that occurred specifically in S. ambofaciens. The two genes are 98% identical, and their deduced protein products exhibit 97% identity at the amino acid level. In contrast with the coding sequences, their genetic environments and their transcriptional control are strongly divergent. While hasL is monocistronic, hasR is arranged in a polycistronic unit with two upstream open reading frames, arsR and prsR, that encode putative anti-anti-and anti-factors, respectively. Transcription of each has gene is initiated from two promoters. In each case, one promoter was shown to be developmentally controlled and to be similar to those recognized by the B. subtilis general stress response sigma factor B. Expression from this type of promoter for each of the has genes dramatically increases during the course of growth in liquid or on solid media and following oxidative and osmotic stresses. Reverse transcription-PCR measurements indicate that hasR is 100 times more strongly expressed than hasL from the B-like promoter. Transcription from the second promoter of each gene (located upstream of arsR in the case of the hasR locus) appears to be constitutive and weak. Quantitative transcriptional analysis in single and double has mutant strains revealed that BR and BL direct their own transcription as well as that of their duplicates. Only a slight sensitivity in response to oxidative conditions could be assigned to either single or double mutants, revealing the probable redundancy of the factors implied in stress response in Streptomyces.