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J. Biol. Chem., Vol. 275, Issue 22, 17058-17063, June 2, 2000

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Mutations That Increase the Activity of the Promoter of the Escherichia coli Melibiose Operon Improve the Binding of MelR, a Transcription Activator Triggered by Melibiose^*

Eiji Tamai, Tamara A. Belyaeva^§, Stephen J. W. Busby^§, and Tomofusa Tsuchiya^¶

From the  Department of Microbiology, Faculty of Pharmaceutical Sciences, Okayama University, Tsushima, Okayama, 700-8530 Japan and the ^§ School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

MelR is an Escherichia coli transcription factor that activates expression of the melAB operon in response to the presence of melibiose in the environment. MelR stimulates transcription initiation at the melAB promoter by binding to four sites centered at positions 120.5, 100.5, 62.5, and 42.5 upstream of the transcript start point. In a previous study, we described a spontaneous mutant that exhibited increased melAB expression. Sequence analysis showed that this mutant carries five consecutive base changes at positions 49, 50, 51, 52, and 53 upstream of the melAB transcript start. Here we show that these changes improve MelR binding to the target site centered at position 42.5 at the melAB promoter and that this improvement is responsible for increased promoter activity. Thus, the activity of the melAB promoter is fixed by the occupation by MelR of a DNA site that overlaps the 35 hexamer: MelR appears to be a typical class II-type transcription activator.

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