Abstract
Two classes of mutations with promoter effects on the biotin ( bio) gene cluster of Escherichia coli have been identified. The first class are point mutations that increase leftward ( bioA) transcription four- to sixfold and that diminish by two-thirds rightward ( bioB-bioD) transcription. The second class are polar insertions of about 1400 base-pairs in the bio operator-promoter region, which prevent transcription in both directions. Four other mutants were selected as operator-constitutive for leftward transcription of galK in a strain where galK is fused by deletion to the bioA transcript. These mutants are also constitutive for rightward transcription. Thus, leftward and rightward transcription are not under completely independent control. The results are formally satisfied if the two transscripts physically overlap, with operators controlling both transcripts in the overlap region.
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