NEGATIVE TRANSCRIPTIONAL REGULATION OF A POSITIVE REGULATOR - THE EXPRESSION OF MALT, ENCODING THE TRANSCRIPTIONAL ACTIVATOR OF THE MALTOSEREGULON OF ESCHERICHIA-COLI, IS NEGATIVELY CONTROLLED BY MLC

The maltose regulon consists of 10 genes encoding a multicomponent and binding protein-dependent ABC transporter for maltose and maltodextri ns as well as enzymes necessary for the degradation of these sugars. M alT, the transcriptional activator of the system, is necessary for the transcription of all mal genes. MalK, the energy-transducing subunit of the transport system, acts phenotypically as repressor, particularl y when overproduced. We isolated an insertion mutation that strongly r educed the repressing effect of overproduced MalK. The affected gene w as sequenced and identified as mlc, a known gene encoding a protein of unknown function with homology to the Escherichia coil NagC protein. The loss of Mlc function led to a threefold increase in malT expressio n, and the presence of mle on a multicopy plasmid reduced malT express ion. By DNasel protection assay, we found that Mlc protected a DNA reg ion comprising positions +1 to +23 of the malT transcriptional start p oint. Using a mlc-lacZ fusion in a mle and mlc(+) background, we found that Mlc represses its own expression. As Mlc also regulates another operon (manXYZ, see pages 369-379 of this issue), it may very well con stitute a new global regulator of carbohydrate utilization.