The salIR and salIM genes of Streptomyces albus G encode the restriction endonuclease (ENase) and DNA methyltransferase (MTase) of the SalI restriction-modification (R-M) system. In S. albus G, the genes constitute an operon that is mainly transcribed from a promoter located upstream from salIR, the first gene of the operon. In addition, a second promoter, at the 3' end of salIR, allows independent transcription of the MTase gene. Expression of salIR and salIM in Escherichia coli was investigated. The ENase gene was not expressed in the heterologous host, probably due to inactivity of the main promoter of the salI operon. In contrast to salIR, salIM was functional in E. coli. Preliminary S1 nuclease mapping experiments suggest that the alternative promoter of the MTase gene can initiate transcription in the heterologous, as well as in the homologous host.