Sequence and properties of operator mutations in the bio operon of Escherichia coli

Abstract

The nucleotide-sequence changes occurring in newly isolated operator-constitutive mutations of the divergently transcribed bio operon have been determined. The observed point mutations are single GC → AT changes which occur at two symmetrical points in the hyphenated inverted repeat present in the control region. The changes at position -15 (with respect to the center of the inverted repeat) cause constitutivity of leftward operon expression and decreased expression of bioB, due to alteration of the -35 region of the rightward promoter. The change at position +15 is identical to one of the changes that Otsuka and Abelson (1978) detected in the bio p98 bio o34 double mutant. The location of the bio p 131:IS1 insertion, which affects both leftward and rightward transcription, is also within the operator region. Both of the operator-constitutive mutations and the bio p131 insertion cause decreased represser binding in vivo as shown by repressor titration tests on multicopy plasmids which bear them. The operator-constitutive mutations also decrease repressor binding in vitro, where added repressor fails to protect the TaqI site that is protected by repressor in bio o + DNA. These results confirm several aspects of the model for bio operon regulation proposed by Otsuka and Abelson (1978).