Two improved promoter sequences for the beta-lactamase expression arising from a single base-pair substitution.

A mutation in the transposon Tn2660 derived from the plasmid R6K, and resulting in an approximate tenfold increase in penicillin resistance is shown to be a single GC to AT substitution 177 base pairs 'upstream' of the initiation codon of the structural beta-lactamase (bla) gene. This substitution leads to the transcription of two new mRNAs which can be ascribed to the creation of two new overlapping promoter sequences. All the sequences (450bp) examined in the wild-type Tn2660 are identical to those reported in Tn3.