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Microbiol. Rev., 12 1995, 623-645, Vol 59, No. 4
C Condon, C Squires and CL Squires
The control of rRNA synthesis in response to both extra- and intracellular
signals has been a subject of interest to microbial physiologists for
nearly four decades, beginning with the observations that Salmonella
typhimurium cells grown on rich medium are larger and contain more RNA than
those grown on poor medium. This was followed shortly by the discovery of
the stringent response in Escherichia coli, which has continued to be the
organism of choice for the study of rRNA synthesis. In this review, we
summarize four general areas of E. coli rRNA transcription control:
stringent control, growth rate regulation, upstream activation, and
anti-termination. We also cite similar mechanisms in other bacteria and
eukaryotes. The separation of growth rate-dependent control of rRNA
synthesis from stringent control continues to be a subject of controversy.
One model holds that the nucleotide ppGpp is the key effector for both
mechanisms, while another school holds that it is unlikely that ppGpp or
any other single effector is solely responsible for growth rate-dependent
control. Recent studies on activation of rRNA synthesis by cis-acting
upstream sequences has led to the discovery of a new class of promoters
that make contact with RNA polymerase at a third position, called the UP
element, in addition to the well-known -10 and -35 regions. Lastly, clues
as to the role of antitermination in rRNA operons have begun to appear.
Transcription complexes modified at the antiterminator site appear to
elongate faster and are resistant to the inhibitory effects of ppGpp during
the stringent response.
Copyright © 1995, American Society for Microbiology
Control of rRNA transcription in Escherichia coli
Department of Molecular Biology and Microbiology, Tufts University Health Sciences Campus, Boston, Massachusetts 02111, USA.
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