Anomalous expression of the E. coli lac operon in Proteus mirabilis. I. Effects of L8 and L8 UV5.

The lac operon shows anomalous expression in Proteus mirabilis: the maximal induced level is 10% or less of that in E. coli, while repression reduces this by a factor of only 2-5. We have sought to determine whether this effect relates in any way to CRP-mediated activation of expression, by comparing expression in P. mirabilis of lac operons (introduced for technical reasons on IncP1 plasmids) either regulatorily wild-type or bearing L8 or L8UV5. Derivatives of RP1 bearing L8UV5 were obtained by homogenotisation of pGC9114 (RP1::Tn951) in a L8UV5 background; while derivatives of RP4 bearing lac+, L8 or L8UV5 were obtained by Mu-mediated translocation of chromosomal regions bearing these alleles, following partial heat-induction of Mucts62 on pGM14 (RP4::Mucts62) in the appropriate hosts. These plasmids could be readily transferred to, and stably maintained in, the P. mirabilis strains employed. It was found that L8 reduced the maximal level of beta-galactosidase activity, and L8UV5 restored this activity to around wild-type, in P. mirabilis quantitatively very much as in E. coli. Nevertheless, the low maximal level of expression and high basal level characteristic of the former host were unchanged. The simplest explanation of these results is that P. mirabilis contains a protein that mimics the E. coli CRP protein in interacting with the appropriate DNA binding site and thereby stimulating transcription; and that the anomalous regulation of lac in this host is unconnected with the CRP system.