An oligoribonuclease gene in Streptomyces griseus

In Streptomyces griseus, A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-bu tyrolactone) serves as a microbial hormone that switches on many genes requ ired for streptomycin production and morphological development. An open rea ding frame (Orf1) showing high sequence similarity to oligoribonucleases of various origins is present just downstream of adpA, one of the A-factor-de pendent genes. Orf1 was named OrnA (oligoribonuclease A) because it showed 3'-to-5' exo-oligoribonuclease activity, releasing [P-32] CMP from ApCpC [P -32] PC used as a substrate. Reverse transcription-PCR and S1 nuclease mapp ing analyses revealed that ornA was transcribed from two promoters; one was a developmentally regulated, A-factor-dependent promoter in front of adpA, and the other was a constitutive promoter in front of the ornA coding sequ ence. Transcription of ornA was thus additively enhanced at the initiation stage for secondary metabolism and aerial mycelium formation. ornA-disrupte d strains grew slowly and scarcely formed aerial mycelium. ornA homologues were distributed in a wide variety of Streptomyces species, including S. co elicolor A3(2), as determined by Southern hybridization analysis. Disruptio n of the ornA homologue in S. coelicolor A3(2) also caused phenotypes simil ar to those of the S. griseus Delta ornA strains. The OrnA oligoribonucleas es in Streptomyces species are therefore not essential but play an importan t role in vegetative growth and in the initiation of differentiation.