Differential expression of two sporulation specific sigma factors of Streptomyces aureofaciens correlates with the developmental stage

In previous experiments, the Streptomyces aureofaciens (Sa) rpoZ, and sigF genes have been shown to encode putative sigma factors essential in differentiation. In an attempt to investigate expression of these genes during differentiation, we have performed S1 nuclease mapping using RNA prepared from Sa in various developmental stages. Two putative promoters were identified upstream of the rpoZ coding region. The promoters significantly differed in their strength, and were active in distinct developmental stages; the weaker, rpoZ-P1, was active only in substrate mycelium, and the stronger, rpoZ-P2, was induced at the beginning of aerial mycelium formation. Transcriptional analysis of sigF revealed two apparent transcription start points, both being detectable only in the late phase of aerial mycelium formation. No sigF transcription was-detected in an rpoZ-disrupted Sa strain. Promoter-bearing DNA fragments from rpoZ and sigF were inserted into several promoter-probe vectors, to give expression patterns consistent with the results of direct RNA analysis. The results implicate temporally different expression of rpoZ and sigF during the differentiation of Sa, and direct or indirect dependence of sigF expression on the rpoZ-encoded putative sigma factor, thus indicating a cascade of sigma factors in Streptomyces development.