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Mol. Cell. Biol., Oct 1997, 6114-6121, Vol 17, No. 10
PJ Wang, A Chabes, R Casagrande, XC Tian, L Thelander and TC Huffaker
Ribonucleotide reductases catalyze the formation of deoxyribonucleotides by
the reduction of the corresponding ribonucleotides. Eukaryotic
ribonucleotide reductases are alpha2beta2 tetramers; each of the larger,
alpha subunits possesses binding sites for substrate and allosteric
effectors, and each of the smaller, beta subunits contains a binuclear iron
complex. The iron complex interacts with a specific tyrosine residue to
form a tyrosyl free radical which is essential for activity. Previous work
has identified two genes in the yeast Saccharomyces cerevisiae, RNR1 and
RNR3, that encode alpha subunits and one gene, RNR2, that encodes a beta
subunit. Here we report the identification of a second gene from this
yeast, RNR4, that encodes a protein with significant similarity to the
beta-subunit proteins. The phenotype of rnr4 mutants is consistent with
that expected for a defect in ribonucleotide reductase; rnr4 mutants are
supersensitive to the ribonucleotide reductase inhibitor hydroxyurea and
display an S-phase arrest at their restrictive temperature. rnr4 mutant
extracts are deficient in ribonucleotide reductase activity, and this
deficiency can be remedied by the addition of exogenous Rnr4p. As is the
case for the other RNR genes, RNR4 is induced by agents that damage DNA.
However, Rnr4p lacks a number of sequence elements thought to be essential
for iron binding, and mutation of the critical tyrosine residue does not
affect Rnr4p function. These results suggest that Rnr4p is catalytically
inactive but, nonetheless, does play a role in the ribonucleotide reductase
complex.
Copyright © 1997, American Society for Microbiology
Rnr4p, a novel ribonucleotide reductase small-subunit protein
Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, New York 14853, USA.
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