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Journal of Bacteriology, September 1999, p. 5684-5692, Vol. 181, No. 18
Institut für Biologie der
Humboldt-Universität zu Berlin, Berlin, Germany
Received 1 February 1999/Accepted 2 July 1999
The protein HoxA is the central regulator of the Alcaligenes
eutrophus H16 hox regulon, which encodes two
hydrogenases, a nickel permease and several accessory proteins required
for hydrogenase biosynthesis. Expression of the regulatory gene
hoxA was analyzed. Screening of an 8-kb region upstream of
hoxA with a promoter probe vector localized four promoter
activities. One of these was found in the region immediately 5' of
hoxA; the others were correlated with the nickel metabolism
genes hypA1, hypB1, and hypX. All
four activities were independent of HoxA and of the minor transcription factor
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Positive Transcriptional Feedback Controls
Hydrogenase Expression in Alcaligenes eutrophus
H16
and
54. Translational fusions revealed that
hoxA is expressed constitutively at low levels. In contrast
to these findings, immunoblotting studies revealed a clear fluctuation
in the HoxA pool in response to conditions which induce the
hox regulon. Quantitative transcript assays indicated elevated levels of hyp mRNA under hydrogenase-derepressing
conditions. Using interposon mutagenesis, we showed that the activity
of a remote promoter is required for hydrogenase expression and
autotrophic growth. Site-directed mutagenesis revealed that
PMBH, which directs transcription of the structural genes
of the membrane-bound hydrogenase, contributes to the expression of
hoxA under hydrogenase-derepressing conditions. Thus,
expression of the hox regulon is governed by a positive
feedback loop mediating amplification of the regulator HoxA. These
results imply the existence of an unusually large (ca.
17,000-nucleotide) transcript.
*
Corresponding author. Mailing address: Institut
für Biologie, Mikrobiologie, Humboldt-Universität zu
Berlin, Chausseestr. 117, D-10115 Berlin, Germany. Phone:
49-30-2093-8117. Fax: 49-30-2093-8102. E-mail:
edward.schwartz{at}rz.hu-berlin.de.
Present address: Abteilung Angewandte Mikrobiologie,
Universität Ulm, D-89069 Ulm, Germany.
Journal of Bacteriology, September 1999, p. 5684-5692, Vol. 181, No. 18
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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