Comparative analysis of expression of the SalI restriction-modification system in Escherichia coli and Streptomyces.

The salIR and salIM genes encode the endonuclease and methyltransferase components of the SalI restriction-modification system from Streptomyces albus G. Expression of the salI genes in Escherichia coli was investigated and major differences with Streptomyces were found. In E. coli there is no detectable expression of the salI R gene due to inactivity of the sal-pR promoter region. In the natural host of the system this region directs transcription of the salI genes as a bicistronic message. In contrast to salIR, salIM is transcribed in the heterologous host from a promoter within the salI DNA. Since sal-pR is not active, the gene cannot be expressed as part of the salI operon. It is probably transcribed from sal-pM, a promoter internal to the operon which allows independent expression of the modification gene in Streptomyces. Replacement of sal-pR by the strong pLac promoter allows expression of salIR in E. coli and enhances expression of salIM. The resulting strain produces about 10 times more endonuclease than a Streptomyces clone containing the SalI system under the control of sal-pR.