Anomalous expression of the E. coli lac operon in Proteus mirabilis. II. Effects of lacI and lacP mutations.

The lac operon introduced into Proteus mirabilis shows two anomalies of expression: the maximal induced level is reduced by about an order of magnitude, and the basal level becomes about 100 times higher than in E. coli, as a result of which the induction ratio appears very small, around 2-5 in contrast to as much as 1000 in E. coli. It was suggested by Baumberg and Dennison (1975) that the two anomalies might be manifestations of a single effect whereby some exogenous promoters give poor expression in this host, since they could result from poor expression of the lacZYA and lacI promoters respectively. We show here that when the lacP class II promoter mutation L305 was transferred on an F-prime into P. mirabilis, its effect on lac expression was much as in E. coli. However, when Flac bearing the IQ1 up-promoter mutation was introduced into P. mirabilis, the basal level decreased by three orders of magnitude, in accord with poor expression of lacI+ being responsible for the usual low induction ratio. These results are consistent with the hypothesis of Baumberg and Dennison (1975) but do not prove it: poor expression of lacI and/or lacZYA could also result from weak translation initiation, problems of continuation of transcription or translation (e.g. due to pause sequences or differences in codon use), or diminished mRNA stability.