NOVEL GROWTH-RATE CONTROL OF DAM GENE-EXPRESSION IN ESCHERICHIA-COLI

Transcription of the dam gene in Escherichia coil is growth rate regul ated by a mechanism distinct from that used for ribosomal RNA gene pro moters. Single-copy operon fusions to lacZ indicated that the major pr omoter, P2, is responsible for most or all of the growth rate dependen ce. Promoter P2 is a typical sigma(70) promoter with 18 bp spacing bet ween the -10 and -35 hexamers. Primer extension analysis was used to s how that there was no inhibition of transcription from promoter P2 in eels induced for the stringent response. Beta-galactosidase specific a ctivity from a single-copy dam::lacZ fusion was unaffected by either e xcess rrnB RNA or the level of Fis protein. Thus growth rate control o f dam gene expression differs from that of the rRNA and tRNA genes by its lack of response to stringent control, ribosomal feedback and enha nced transcription by Fis protein. We devised a procedure for selectio n of mutant cells in which dam gene expression was unregulated. One su ch mutant (cde-4), obtained by miniTn10 insertion, showed the same lev el of beta-galactosidase activity at all growth rates tested. In contr ast, growth rate-dependent expression of the rrnB gene was unaffected by cde-4 confirming the different modes of regulation. The cde-4::mini Tn10 insertion is located close to kilobase 670 on the physical map in or near the lipB gene.