title: The Ser176 of T4 endonuclease IV is crucial for the restricted and polarized dC-specific cleavage of single-stranded DNA implicated in restriction of dC-containing DNA in host Escherichia coli.
abstract: Endonuclease (Endo) IV encoded by denB of bacteriophage T4 is an enzyme that cleaves single-stranded (ss) DNA in a dC-specific manner. Also the growth of dC-substituted T4 phage and host Escherichia coli cells is inhibited by denB expression presumably because of the inhibitory effect on replication of dC-containing DNA. Recently, we have demonstrated that an efficient cleavage by Endo IV occurs exclusively at the 5'-proximal dC (dC1) within a hexameric or an extended sequence consisting of dC residues at the 5'-proximal and the 3'-proximal positions (dCs tract), in which a third dC residue within the tract affects the polarized cleavage and cleavage rate. Here we isolate and characterize two denB mutants, denB(W88R) and denB(S176N). Both mutant alleles have lost the detrimental effect on the host cell. Endo IV(W88R) shows no enzymatic activity (<0.4% of that of wild-type Endo IV). On the other hand, Endo IV(S176N) retains cleavage activity (17.5% of that of wild-type Endo IV), but has lost the polarized and restricted cleavage of a dCs tract, indicating that the Ser176 residue of Endo IV is implicated in the polarized cleavage of a dCs tract which brings about a detrimental effect on the replication of dC-containing DNA.
authors.affiliation: Department of Applied Biological Science, Nihon University College of Bioresource Sciences, Fujisawa-shi, Kanagawa 252-8510, Japan.
authors.forename: Nobutaka
authors.initials: N
authors.lastname: Hirano
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Ohshima
authors.forename: Hidenori
authors.initials: H
authors.lastname: Sakashita
authors.forename: Hideo
authors.initials: H
authors.lastname: Takahashi
mesh_terms: D017122:Bacteriophage T4 / Q000201:enzymology*; D004269:DNA, Bacterial / Q000378:metabolism*; D004277:DNA, Single-Stranded / Q000378:metabolism*; D043223:Deoxyribonuclease IV (Phage T4-Induced) / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000821:virology*; D017354:Point Mutation; D012694:Serine / Q000378:metabolism
chemical_list: D004269:DNA, Bacterial; D004277:DNA, Single-Stranded; D012694:Serine; D043223:Deoxyribonuclease IV (Phage T4-Induced)