The malPQ operon, one of the three operons of the maltose regulon, is positively controlled by the product of gene malT. The starting point for malPQ transcription was deduced from experiments which involved a hybridization of in vivo-synthesized malPQ mRNA with adequate DNA probes, followed either by a digestion of nonhybridized DNA (S1 nuclease mapping) or by an extension of the hybridized probe (reverse transcriptase mapping). In the wild-type strain, this starting point was 37 nucleotides upstream from the initiation codon for malP. This analysis was also performed on a double mutant which contained both a 13-base pair deletion and a 3-base pair insertion in the promoter region. This double mutant expressed the malPQ operon exactly as the wild-type strain did, in a maltose-inducible manner. In this strain, the starting point for malPQ transcription was shifted 11 nucleotides downstream from the wild-type location. An analysis of these results suggests that (i) the binding site for the malT product is located upstream from the region which is severely altered in the double mutant, i.e., upstream from position -31; and (ii) the 30-base pair sequence which precedes the transcription starting point contains very few positions which are essential for promoter activity.
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