EVIDENCE FOR DISTINCT PRIMASE AND HELICASE DOMAINS IN THE 63-KDA GENE-4 PROTEIN OF BACTERIOPHAGE-T7 - CHARACTERIZATION OF A NUCLEOTIDE-BINDING SITE MUTANT
Lv. Mendelman et al., EVIDENCE FOR DISTINCT PRIMASE AND HELICASE DOMAINS IN THE 63-KDA GENE-4 PROTEIN OF BACTERIOPHAGE-T7 - CHARACTERIZATION OF A NUCLEOTIDE-BINDING SITE MUTANT, The Journal of biological chemistry, 268(36), 1993, pp. 27208-27213
Gene 4 of bacteriophage T7 encodes two co-linear proteins, the 56- and
63-kDa gene 4 proteins. The 56-kDa protein translocates 5' to 3' on s
ingle-stranded DNA using nucleotide hydrolysis for energy and is a hel
icase. The 63-kDa gene 4 protein catalyzes all of the activities of th
e 56-kDa gene 4 protein and, in addition, catalyzes the synthesis of o
ligoribonucleotides on single-stranded DNA. Two conserved residues in
a putative nucleotide binding site of the 63-kDa gene 4 protein were m
utated by substituting Val and Met for wild-type residues Gly and Lys,
at positions 317 and 318, respectively. The mutant 63-kDa gene 4 prot
ein lacks the ability to catalyze the hydrolysis of a nucleoside 5'-tr
iphosphate in a single-stranded DNA-dependent reaction and inhibits nu
cleotide hydrolysis by wild-type gene 4 proteins. The mutant primase c
ontains 0.4% of the primase activity of the 63-kDa gene 4 protein on M
13 single-stranded DNA and 12% of the wild-type primase activity on an
oligonucleotide with a single primase recognition site. Addition of w
ild-type 56-kDa gene 4 protein stimulates the mutant primase activity
over 50-fold on M13 single-stranded DNA and 8-fold on oligonucleotides
. This increase in primase activity correlates with an increase in the
affinity of the mutant primase-wild-type helicase complex for single-
stranded DNA template.