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

Wang, Xun, Ji, Sang Chun, Jeon, Heung Jin, Lee, Yonho, Lim, Heon M.
Proceedings of the National Academy of Sciences of the United States of America 2015 v.112 no.24 pp. 7581-7586
Escherichia coli, messenger RNA, operon
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 .