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(Received for publication, June 24, 1997, and in revised form, September 22, 1997)
From the Max-Planck-Institut für Molekulare Genetik, Dahlem,
Ihnestrasse 73, D-14195 Berlin, Germany and The RepA protein of the mobilizable broad host
range plasmid RSF1010 has a key function in its replication. RepA is
one of the smallest known helicases. The protein forms a homohexamer of
29,896-Da subunits. A variety of methods were used to analyze the
quaternary structure of RepA. Gel filtration and cross-linking experiments demonstrated the hexameric structure, which was confirmed by electron microscopy and image reconstruction. These results agree
with recent data obtained from RepA crystals diffracting at 3.5-Å
resolution (Röleke, D., Hoier, H., Bartsch, C., Umbach, P.,
Scherzinger, E., Lurz, R., and Saenger, W. (1997) Acta
Crystallogr. Sec. D 53, 213-216). The RepA helicase has 5
Volume 272, Number 48,
Issue of November 28, 1997
pp. 30228-30236
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
The RepA Protein of Plasmid RSF1010 Is a Replicative DNA
Helicase
,
,
Centro
Nacional de Biotecnologia-CSIC, Campus Universidad Autónoma,
Cantoblanco, 28049 Madrid, Spain
3
polarity. As do most true replicative helicases, RepA prefers a
tailed substrate with an unpaired 3
-tail mimicking a replication fork.
Optimal unwinding activity was achieved at the remarkably low pH of
5.5. In the presence of Mg2+ (Mn2+) ions, the
RepA activity is fueled by ATP, dATP, GTP, and dGTP and less
efficiently by CTP and dCTP. UTP and dTTP are poor effectors. Nonhydrolyzable ATP analogues, ADP, and pyrophosphate inhibit the
helicase activity, whereas inorganic phosphate does not. The presence
of Escherichia coli single-stranded DNA-binding protein stimulates unwinding at physiological pH 2-3-fold, whereas the RSF1010
replicon-specific primase, RepB
protein, has no effect, either in the
presence or in the absence of single-stranded DNA-binding protein.
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