COMPARATIVE-ANALYSIS OF EXPRESSION OF THE SAL-I RESTRICTION-MODIFICATION SYSTEM IN ESCHERICHIA-COLI AND STREPTOMYCES

The salIR and salIM genes encode the endonuclease and methyltransferas e components of the SalI restriction-modification system from Streptom yces albus G. Expression of the sail genes in Escherichia coli was inv estigated and major differences with Streptomyces were found. In E. co li there is no detectable expression of the salIR gene due to inactivi ty of the sal-pR promoter region. In the natural host of the system th is region directs transcription of the sail genes as a bicistronic mes sage. In contrast to salIR, salIM is transcribed in the heterologous h ost from a promoter within the salI DNA. Since sal-pR is not active, t he gene cannot be expressed as part of the sail operon. It is probably transcribed from sal-pM, a promoter internal to the operon which allo ws independent expression of the modification gene in Streptomyces. Re placement of sal-pR by the strong pLac promoter allows expression of s alIR in E. coli and enhances expression of salIM. The resulting strain produces about 10 times more endonuclease than a Streptomyces clone c ontaining the SalI system under the control of sal-pR.