Negative Transcriptional Regulation of a Positive Regulator: The Expression of malT, Encoding the Transcriptional Activator of the Maltose Regulon of Escherichia Coli, Is Negatively Controlled by Mlc

Mol Microbiol. 1998 Jan;27(2):381-90. doi: 10.1046/j.1365-2958.1998.00694.x.


The maltose regulon consists of 10 genes encoding a multicomponent and binding protein-dependent ABC transporter for maltose and maltodextrins as well as enzymes necessary for the degradation of these sugars. MalT, 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, particularly when overproduced. We isolated an insertion mutation that strongly reduced the repressing effect of overproduced MalK. The affected gene was sequenced and identified as mlc, a known gene encoding a protein of unknown function with homology to the Escherichia coli NagC protein. The loss of Mlc function led to a threefold increase in malT expression, and the presence of mlc on a multicopy plasmid reduced malT expression. By DNasel protection assay, we found that Mlc protected a DNA region comprising positions +1 to +23 of the malT transcriptional start point. Using a mlc-lacZ fusion in a mlc 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 constitute a new global regulator of carbohydrate utilization.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • ATP-Binding Cassette Transporters*
  • Bacterial Proteins / biosynthesis
  • Bacterial Proteins / genetics*
  • Carrier Proteins / biosynthesis
  • Carrier Proteins / genetics
  • Cystathionine gamma-Lyase / biosynthesis
  • Cystathionine gamma-Lyase / genetics
  • DNA Transposable Elements
  • DNA-Binding Proteins*
  • Escherichia coli / genetics*
  • Escherichia coli / metabolism
  • Escherichia coli Proteins*
  • Gene Expression Regulation, Bacterial*
  • Homeostasis
  • Kanamycin Resistance / genetics
  • Maltose / metabolism*
  • Mutagenesis, Insertional
  • Regulatory Sequences, Nucleic Acid
  • Regulon*
  • Repressor Proteins / genetics
  • Repressor Proteins / metabolism*
  • Trans-Activators / biosynthesis
  • Trans-Activators / genetics*
  • Transcription Factors*
  • Transcription, Genetic*


  • ATP-Binding Cassette Transporters
  • Bacterial Proteins
  • Carrier Proteins
  • DNA Transposable Elements
  • DNA-Binding Proteins
  • Escherichia coli Proteins
  • MalK protein, Bacteria
  • MalK protein, E coli
  • MalT protein, E coli
  • Mlc protein, E coli
  • Repressor Proteins
  • Trans-Activators
  • Transcription Factors
  • MalY protein, E coli
  • Maltose
  • Cystathionine gamma-Lyase