Properties of several single-stranded DNA phages were investigated with special reference to their host factor requirements. Phages ~5B and G6 were inactivated by anti-cX174 serum but at different rates. Although host ranges were very similar, plating efficiencies of the two phages differed from each other on E. coli CR63.1 as well as on C600 1. Phage ¢C was immunologically remote from ~5B and G6 and, like G4, able to infect C/15. At 42°, ~5B and G6 could replicate in E. coli dna+ cells,

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... growth of G4, G13, G l4, and U3 was restricted under this condition. Growth of ¢C was more thermosensitive and the free-phage yield at 35° was only 0.1 % of the yield at 30°. Unlike ~bA, replication of G4, ¢C, and U3 proceeded at 22° rather normally, whereas growth of ~5B and G6 was considerably retarded at this low temperature. In dna+ hcr+ hosts, multiplication of G6 and ¢B was inhibited by 1 to 2 mg/ml of hydroxyurea, 0.3 % phenethyl alcohol, 10 ug/ml of mitomycin C, or 20 sg/ml of nalidixic acid. In strain C infected with G6 or OB, formation of the parental RF occurred in the presence of 200 ,ug/ml of rifampicin, 50 ,ug/ml of nalidixic acid, 50 pg/ml of mitomycin C, or 25 , tg/ml of chloramphenicol. Transfection experiments showed that replication of G6 depended on host functions determined by dnaB, -C(D), -E, -G, and -Z, as well as by ligts7 and rep3. The host products specified by these genes were also indispensable for ~SB. Although several q5X174-type SS DNA phages have hitherto been described, relatively little is known about the evolutionary relationship among them. BRAD-LEY (1) has classified the icosahedral SS DNA phages into three groups by the host range and the antigenicity; group A (e.g., a3) can replicate in E. coli B and C, Abbreviations used: PFU, plaque-forming unit. RF, double-stranded replicative form DNA; SS, single-stranded DNA; 85 TAKETO Vol. 23 Replicative Properties of ~5X-type Phages 93