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J Bacteriol, June 1998, p. 2889-2894, Vol. 180, No. 11
Consejo Superior de Investigaciones
Científicas, Estación Experimental del Zaidín,
Department of Biochemistry, E-18008 Granada,
Spain,1 and
GBF, D-38124 Braunschweig,
Germany2
Received 9 January 1998/Accepted 23 March 1998
The xylR and xylS genes are divergent and
control transcription of the TOL plasmid catabolic pathways for toluene
metabolism. Four promoters are found in the 300-bp intergenic region:
Pr1 and Pr2 are constitutive
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Activation and Repression of Transcription at the Double Tandem
Divergent Promoters for the xylR and xylS Genes
of the TOL Plasmid of Pseudomonas putida
70-dependent tandem
promoters that drive expression of xylR, while expression
of the xylS gene is driven from Ps2, a constitutive
70-dependent promoter, and by the regulatable
54 class Ps1 promoter. In Ps1 the XylR targets (upstream
activator sequences [UASs]) overlap the Pr promoters, and two sites
for integration host factor (IHF) binding are located at the region from positions
2 to
30 (
2/
30 region) and the
137/
156
region, the latter overlapping the Pr promoters. When the XylR protein binds to the UASs in the absence of effector, it represses expression from Pr promoters. In the XylR-plus background and in the absence of an
effector, the level of expression from Ps1 is low, although detectable,
whereas Ps2 is active. In this background and in the presence of an
effector, XylR increases autorepression. In a
54-deficient Pseudomonas putida background,
no expression occurred from Ps1 regardless of the presence of an
effector. However, in the presence of an effector, the amount of RNA
produced from Pr promoters was almost undetectable. This finding
suggests that when no transcription occurred at the Ps1 promoter,
clearance of XylR from the UASs was almost negligible. In this
background, expression from Ps2 was very high regardless of the
presence of an effector; this finding suggests that RNA polymerase
containing
54 modulates expression from the downstream
Ps2
70-dependent promoter. In a P. putida IHF-minus background and in the presence of effector, Ps1
expression was the highest found; in contrast, the basal levels of this
promoter were the lowest observed. This finding suggests that IHF acts
in vivo as a repressor of the
54-dependent Ps1 promoter.
In an IHF-deficient host background, expression from Ps2 in the
presence of effector was negligible. Thus, binding of RNA polymerase
containing
54 at the upstream promoter may modulate
expression from the Ps2 promoter.
*
Corresponding author. Mailing address:
CSIC-Estación Experimental del Zaidín, Apdo Correos 419, E-18008 Granada, Spain. Phone: 34-58-121011. Fax: 34-58-129600. E-mail:
jlramos{at}eez.csic.es.
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