Affiliation:
1. Department of Biochemistry, University of Dundee, Dundee DD1 5EH, Scotland,1 and
2. Nitrogen Fixation Laboratory, John Innes Centre, Norwich NR4 7UH,2 United Kingdom
Abstract
ABSTRACT
The expression of the
moa
locus, which encodes enzymes required for molybdopterin biosynthesis, is enhanced under anaerobiosis but repressed when the bacterium is able to synthesize active molybdenum cofactor. In addition,
moa
expression exhibits a strong requirement for molybdate. The molybdate enhancement of
moa
transcription is fully dependent upon the molybdate-binding protein, ModE, which also mediates molybdate repression of the
mod
operon encoding the high-affinity molybdate uptake system. Due to the repression of
moa
in molybdenum cofactor-sufficient strains, the positive molybdate regulation of
moa
is revealed only in strains unable to make the active cofactor. Transcription of
moa
is controlled at two sigma-70-type promoters immediately upstream of the
moaA
gene. Deletion mutations covering the region upstream of
moaA
have allowed each of the promoters to be studied in isolation. The distal promoter is the site of the anaerobic enhancement which is Fnr-dependent. The molybdate induction of
moa
is exerted at the proximal promoter. Molybdate-ModE binds adjacent to the −35 region of this promoter, acting as a direct positive regulator of
moa
. The molybdenum cofactor repression also appears to act at the proximal transcriptional start site, but the mechanism remains to be established. Tungstate in the growth medium affects
moa
expression in two ways. Firstly, it can act as a functional molybdate analogue for the ModE-mediated regulation. Secondly, tungstate brings about the loss of the molybdenum cofactor repression of
moa
. It is proposed that the tungsten derivative of the molybdenum cofactor, which is known to be formed under such conditions, is ineffective in bringing about repression of
moa
. The complex control of
moa
is discussed in relation to the synthesis of molybdoenzymes in the bacterium.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
65 articles.
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