Identification and in vivo functional analysis of a virginiamycin S resistance gene (varS) from Streptomyces virginiae

BarA of Streptomyces virginiae is a specific receptor protein for virginiae butanolide (VB), one of the gamma-butyrolactone autoregulators of the Stre ptomyces species, and acts as a transcriptional regulator controlling both virginiamycin production and VB biosynthesis. The downstream gene barB, the transcription of which is under the tight control of the VB-BarA system, w as found to be transcribed as a polycistronic mRNA with its downstream regi on, and DNA sequencing revealed a 1,554-bp open reading frame (ORF) beginni ng at 161 bp downstream of the barB termination codon. The ORF product show ed high homology (68 to 73%) to drug efflux proteins having 14 transmembran e segments and was named varS (for S. virginiae antibiotic resistance). Het erologous expression of varS with S. lividans as a host resulted in virgini amycin S-specific resistance, suggesting that varS encoded a virginiamycin S-specific transport protein. Northern blot analysis indicated that the bic istronic transcript of barB-varS appeared 1 to 2 h before the onset of virg iniamycin M-1 and S production, at which time Vp was produced, while exogen ously added virginiamycin S apparently induced the monocistronic varS trans cript.