Mechanism of DNA chain growth. XII. Asymmetry of replication of P2 phage DNA.

Abstract

Abstract [3H]thymidine incorporated into replicating P2 DNA during a brief pulse is found mainly in two size classes of DNA chains: short chains of about 10 S, and long chains of one genome length (32 S). The pulse-labelled 10 S short chains hybridize predominantly with the L-strand of the phage DNA, whereas the pulse-labelled genome-length DNA chains hybridize preferentially with the H-strand. From the direction of replication, the polarity of transcription and the data on hybridization of pulse-labelled RNA with the DNA strands, the over-all directions of elongation of the H and L-strands are deduced to be 3′ → 5′ and 5′ → 3′, respectively. Therefore, in the replication fork of P2 DNA discontinuities occur predominantly in the nascent H-strand elongating in the 3′ → 5′ direction. A small portion of the pulse label is always found in linear L-strand molecules that are longer than one genome length but shorter than two. The label is also incorporated into double-stranded closed circular molecules; the H-strand of these molecules is labelled earlier than their L-strand. The H-strand of the parental phage DNA appears to be conserved as a circle during phage DNA synthesis. These findings are interpreted in terms of a modification of the rolling circle model, which assumes periodic production of a transient nick at the base of the tail in the strand that elongates to more than genome length (the L-strand).