The process of infection with bacteriophage φX174: XXIX. In vivo studies on the synthesis of the single-stranded DNA of progeny φX174 bacteriophage

Abstract

Double-stranded φXDNA rings, composed of a closed complementary strand and an open viral strand, are found in φX-infected cells during the period of progeny single-stranded DNA formation. The linear viral strand components of these double-stranded circles are nicked between a deoxyguanosine (on the 3′-terminus) and a deoxycytidine (on the 5′-terminus). The 5′-terminus is not phosphorylated. Upon replication, nucleotides are added to the 3′-end of the viral strand. The 5′ end is concomitantly displaced from its complementary strand template and appears, by electron microscopy and by column chromatography, as a single-stranded, non-hydrogen-bonded, tail of a double-stranded circle. While the viral strand grows (to a maximum of twice the length of one φX genome) by addition of nucleotides to the 3′-end, an interesting reaction, of uncertain nature, also takes place at the free 5′-end of the nascent viral strand, the net result of which is that the original terminal deoxycytidine is replaced by a terminal deoxyguanosine. Our experiments indicate that the single-stranded DNA synthesis does not take place at the cell membrane where double-stranded φX DNA is formed but upon “cytoplasmic” RF ‡ ‡ Abbreviations used: RF, the double-stranded replicative form DNA of φX. In RFI (super-coiled) both strands are circular and closed. In RFII one or both strands of the circle are nicked. All the RFII molecules described in this paper are composed of a closed circular complementary strand and a nicked viral strand. . Viral coat protein can be shown to be associated with the single-strand-producing complexes.