Two-level inhibition of galK expression by Spot 42: Degradation of mRNA mK2 and enhanced transcription termination before the galK gene

Proc Natl Acad Sci U S A. 2015 Jun 16;112(24):7581-6. doi: 10.1073/pnas.1424683112. Epub 2015 Jun 4.


The Escherichia coli gal operon has the structure Pgal-galE-galT-galK-galM. During early log growth, a gradient in gene expression, named type 2 polarity, is established, as follows: galE > galT > galK > galM. However, during late-log growth, type 1 polarity is established in which galK is greater than galT, as follows: galE > galK > galT > galM. We found that type 2 polarity occurs as a result of the down-regulation of galK, which is caused by two different molecular mechanisms: Spot 42-mediated degradation of the galK-specific mRNA, mK2, and Spot 42-mediated Rho-dependent transcription termination at the end of galT. Because the concentration of Spot 42 drops during the transition period of the polarity type switch, these results demonstrate that type 1 polarity is the result of alleviation of Spot 42-mediated galK down-regulation. Because the Spot 42-binding site overlaps with a putative Rho-binding site, a molecular mechanism is proposed to explain how Spot 42, possibly with Hfq, enhances Rho-mediated transcription termination at the end of galT.

Keywords: Rho-termination; Spot 42; galactose operon; sRNA; target mRNA degradation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Base Sequence
  • Down-Regulation
  • Escherichia coli K12 / genetics*
  • Escherichia coli K12 / growth & development
  • Escherichia coli K12 / metabolism*
  • Escherichia coli Proteins / metabolism
  • Galactokinase / genetics*
  • Galactokinase / metabolism*
  • Galactose / metabolism
  • Galactosyltransferases / genetics
  • Galactosyltransferases / metabolism
  • Gene Expression Regulation, Bacterial
  • Gene Expression Regulation, Developmental
  • Gene Expression Regulation, Enzymologic
  • Genes, Bacterial*
  • Host Factor 1 Protein / metabolism
  • Molecular Sequence Data
  • Operon
  • RNA, Bacterial / genetics
  • RNA, Bacterial / metabolism
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • Transcription Termination, Genetic


  • Escherichia coli Proteins
  • Hfq protein, E coli
  • Host Factor 1 Protein
  • RNA, Bacterial
  • RNA, Messenger
  • Rho protein, E coli
  • Galactosyltransferases
  • Galactokinase
  • Galactose