Characterization of virginiamycin S biosynthetic genes from Streptomyces virginiae

Streptomyces virginiae produces -butyrolactone autoregulators (virginiae butanolide, VB), which control the biosynthesis of virginiamycin M1 and S. A 6.3-kb region downstream of the virginiamycin S (VS)-resistance operon in S. virginiae was sequenced, and four plausible open reading frames (ORFs) (visA, 1,260 bp; visB, 1,656 bp; visC, 888 bp; visD, 1209 bp) were identified. Homology analysis revealed significant similarities with enzymes involved in the biosynthesis of cyclopeptolide antibiotics: VisA (53% identity, 65% similarity) to -lysine 2-aminotransferase (NikC) of nikkomycin D biosynthesis, VisB (66% identity, 72% similarity) to 3-hydroxypicolinic acid:AMP ligase of pristinamycin I biosynthesis, VisC (48% identity, 59% similarity) to lysine cyclodeaminase of ascomycin biosynthesis, and VisD (43% identity, 56% similarity) to erythromycin C-22 hydroxylase of erythromycin biosynthesis. Northern blotting as well as high-resolution S1 analysis of the ORFs revealed that they were transcribed as two bicistronic transcripts, namely 3.0-kb visB-visA and another 2.7-kb visC-visD transcript, with promoters locating upstream of visB and visC, respectively. Transcription of the two operons was observed only 1 h after the VB production, which was 2 h before the virginiamycin production. Furthermore, prompt induction of the transcription was observed as a result of external VB addition, suggesting that the expression of the two operons was under the control of VB.