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J Biol Chem, Vol. 273, Issue 10, 5932-5938, March 6, 1998
Purification and Characterization of Bacillus
subtilis PyrR, a Bifunctional pyr mRNA-binding
Attenuation Protein/Uracil Phosphoribosyltransferase
Robert J.
Turner,
Eric R.
Bonner,
Gail K.
Grabner, and
Robert L.
Switzer
From the Department of Biochemistry, University of Illinois,
Urbana, Illinois 61801
Bacillus subtilis PyrR has been shown
to mediate transcriptional attenuation at three separate sites within
the pyrimidine nucleotide biosynthetic (pyr) operon.
Molecular genetic evidence suggests that regulation is achieved by PyrR
binding to pyr mRNA. PyrR is also a uracil
phosphoribosyltransferase (UPRTase). Recombinant PyrR was expressed in
Escherichia coli, purified to homogeneity, physically and
chemically characterized, and examined with respect to both of these
activities. Mass spectroscopic characterization of PyrR demonstrated a
monomeric mass of 20,263 Da. Gel filtration chromatography showed the
native mass of PyrR to be dependent on protein concentration and
suggested a rapid equilibrium between dimeric and hexameric forms. The
UPRTase activity of PyrR has a pH optimum of 8.2. The
Km value for uracil is very pH-dependent; the Km for uracil at pH
7.7 is 990 ± 114 µM, which is much higher than for
most UPRTases and may account for the low physiological activity of
PyrR as a UPRTase. Using an electrophoretic mobility shift assay, PyrR
was shown to bind pyr RNA that includes sequences from its
predicted binding site in the second attenuator region. Binding of PyrR
to pyr RNA was specific and UMP-dependent with
apparent Kd values of 10 and 220 nM in
the presence and absence of UMP, respectively. The concentration of UMP
required for half-maximal stimulation of binding of PyrR to RNA was 6 µM. The results support a model for the regulation of
pyr transcription whereby termination is governed by the
UMP-dependent binding of PyrR to pyr RNA and
provide purified and characterized PyrR for detailed biochemical
studies of RNA binding and transcriptional attenuation.
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.

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Copyright © 1998 by the American Society for Biochemistry and Molecular Biology.
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