The nucleotide sequence of an aminoglycoside phosphotransferase gene ( rph) from Streptomyces ribosidificus (a ribostamycin producer) was determined. Molecular size, amino acid composition and N-terminal amino acid sequence of the purified rph product confirmed the position of the coding region deduced from the nucleotide sequence. The 5 ' region of rph has been tested for its transcriptional controls; high-resolution mung-bean nuclease mapping of in vivo transcripts revealed one major start point, rphSl, controlled by the rphPl promoter. This transcript was also observed in vitro in run-off experiments using purified Streptomyces RNA polymerase. This transcriptional start point coincided with the translational start site, with the mRNA 5' terminus being pppATG. The results of promoter-probing tests and insertion of a transcriptional termination fragment into the rph promoter region have shown that the rphP1 transcript was sufficient and essential for rph expression.