Origin and direction of bacteriophage T4 DNA replication: II. A gradient of marker frequencies in partially replicated t4 DNA as assayed by transformation

One class of small particles of bacteriophage T4 contains incomplete chromosomes which measure two-thirds of the normal T4 chromosome length. These incomplete chromosomes represent continuous, random segments of the T4 genome. After single infection of Escherichia coli, some incomplete chromosomes i...

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Veröffentlicht in:Journal of molecular biology 1971-09, Vol.60 (2), p.213-233
Hauptverfasser: Marsh, R.C., Breschkin, A.M., Mosig, G.
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Sprache:eng
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Zusammenfassung:One class of small particles of bacteriophage T4 contains incomplete chromosomes which measure two-thirds of the normal T4 chromosome length. These incomplete chromosomes represent continuous, random segments of the T4 genome. After single infection of Escherichia coli, some incomplete chromosomes initiate a single, incomplete cycle of replication. The pattern of this partial replication has suggested that replication of incomplete chromosomes starts at a genetically fixed origin and proceeds in one direction to the end of the chromosome (Mosig & Werner, 1969). In the present study, parental and newly synthesized progeny strands of partially replicated incomplete chromosomes were isolated and assayed separately for marker frequencies by transformation. When all markers in the parental strands are recovered with equal frequency, the frequency of markers in the progeny strands decreases with their increasing distance from gene 43 in a clockwise direction on the genetic map. Because an analogous pattern of frequencies of markers from incomplete chromosomes has been observed by Mosig (1970 a) among progeny particles after co-infection with complete and incomplete chromosomes, we conclude that this pattern of replication is not due to a lack of some essential genes on incomplete chromosomes but due to lack of terminal redundancies, resulting in inability to circularize, and to the fact that normal T4 DNA replication starts near the gene for DNA polymerase ( 43) and proceeds only in the clockwise direction on the genetic map. Our experiments show that DNA molecules need not be circular to initiate the first cycle of replication. However, incomplete chromosomes do not replicate repeatedly. It appears that for continuous replication, circularization is required. The region encompassing genes 40 through 42 has the following unique properties which appear to be correlated with its involvement in initiation of DNA replication: (1) under certain lysis conditions, the parental strands containing these genes remain preferentially attached to cellular components; and (2) under non-denaturing isolation conditions, progeny strands containing markers in these genes dissociate from the parental strands while progeny strands in all other genetic regions remain base-paired to the parental strands.
ISSN:0022-2836
1089-8638
DOI:10.1016/0022-2836(71)90289-0