Hypernegative supercoiling inhibits growth by causing RNA degradation.
Transcription-induced hypernegative supercoiling is a hallmark of Escherichia coli topoisomerase I (topA) mutants. However, its physiological significance has remained unclear. Temperature downshift of a mutant yielded transient growth arrest and a parallel increase in hypernegative supercoiling that was more severe with lower temperature. Both properties were alleviated by overexpression of RNase HI. While ribosomes in extracts showed normal activity when obtained during growth arrest, mRNA on ribosomes was reduced for fis and shorter for crp, polysomes were much less abundant relative to monosomes, and protein synthesis rate dropped, as did the ratio of large to small proteins. Altered processing and degradation of lacA and fis mRNA was also observed. These data are consistent with truncation of mRNA during growth arrest. These effects were not affected by a mutation in the gene encoding RNase E, indicating that this endonuclease is not involved in the abnormal mRNA processing. They were also unaffected by spectinomycin, an inhibitor of protein synthesis, which argued against induction of RNase activity. In vitro transcription revealed that R-loop formation is more extensive on hypernegatively supercoiled templates. These results allow us, for the first time, to present a model by which hypernegative supercoiling inhibits growth. In this model, the introduction of hypernegative supercoiling by gyrase facilitates degradation of nascent RNA; overproduction of RNase HI limits the accumulation of hypernegative supercoiling, thereby preventing extensive RNA degradation.
Baaklini
Imad
I
Département de Microbiologie et Immunologie, Université de Montréal, 2900 Edouard-Montpetit, Room R-612, Montréal, Québec H3C 3J7, Canada.
Usongo
Valentine
V
Nolent
Flora
F
Sanscartier
Patrick
P
Hraiky
Chadi
C
Drlica
Karl
K
Drolet
Marc
M
D002373:Cyclic AMP Receptor Protein / Q000235:genetics / Q000378:metabolism; D027081:DNA Gyrase / Q000235:genetics / Q000378:metabolism; D004264:DNA Topoisomerases, Type I / Q000235:genetics / Q000378:metabolism; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004278:DNA, Superhelical / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D035021:Factor For Inversion Stimulation Protein / Q000235:genetics / Q000378:metabolism; D009690:Nucleic Acid Conformation; D010088:Oxidoreductases / Q000235:genetics / Q000378:metabolism; D014176:Protein Biosynthesis; D020871:RNA Stability; D016914:Ribonuclease H / Q000235:genetics / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D013696:Temperature
D002373:Cyclic AMP Receptor Protein; D004269:DNA, Bacterial; D004278:DNA, Superhelical; D029968:Escherichia coli Proteins; D035021:Factor For Inversion Stimulation Protein; C501034:Fis protein, E coli; C481662:crp protein, E coli; D010088:Oxidoreductases; C462500:tRNA-dihydrouridine synthase; D016914:Ribonuclease H; C078856:ribonuclease HI; D004264:DNA Topoisomerases, Type I; D027081:DNA Gyrase
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