COMPLEX TRANSCRIPTION OF AN OPERON ENCODING THE SALI RESTRICTION MODIFICATION SYSTEM OF STREPTOMYCES-ALBUS-G

High-resolution S1 nuclease mapping of mRNA synthesised in vivo, in vi tro run-off transcription with RNA polymerase from Streptomyces livida ns and gene fusions were used to analyse the transcriptional organizat ion of the SalI restriction-modification system of Streptomyces albus G. The sallR and sallM genes that encode the restriction endonuclease and its cognate methyltransferase constitute an operon which is mainly transcribed from sal-pR1, a promoter located immediately upstream of sallR, with two possible minor promoters further upstream. Another pro moter, sal-pM, is within the 3' end of the sallR coding region, and al lows expression of the modification gene in the absence of sal-pR1. Th e sal-pM promoter might be involved in the establishment of modificati on prior to restriction endonuclease activity. Sequences upstream of t he apparent transcriptional start sites for sal-pR1 and sal-pM show si milarity with the -10 region of typical vegetatively expressed eubacte rial promoters, but appropriately centered -35 regions are absent.