lac Promoter mutations located downstream from the transcription start site.

lac promoter mutations which help to define the degree of overlap between the promoter and operator have been isolated and characterized. lacPr115 partially relieves the dependence of lac expression on the catabolite gene activator protein. P'11.5 is an A/T + T/A transversion at base-pair + 1 (the site of transcription initiation in vitro) that has no effect on the repressor-operator interaction. S7 is a mutation isolated and sequenced by L. Johnsrud & J. Miller (personal communication) located at base-pair +6. It exhibits a partial cata-bolite gene activator protein-independent phenotype and an 0 " phenotype. Prl 11 and P'112 are C/G-f A/T transversions at base-pair + 10. P'lll and P'112 also generate a partrial catabolite gene activator protein-independent and 0 " phenotype. Thus, sequences at and downstream from the transcription start site play a role in determining the rate of a functional RNA polymerase-promoter interaction.