Affiliation:
1. Département de Microbiologie et Immunologie, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, Québec H3C 3J7, Canada
2. Public Health Research Institute, New Jersey Medical School, Newark, New Jersey 07103
Abstract
ABSTRACT
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.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
34 articles.
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