Endonucleolytic cleavage of parental DNA and T4 late-gene expression: distribution analysis of single-strand and double-strand breaks.

In order to investigate the dependency of late transcription on concurrent DNA replication during bacteriophage T4 development, we analyzed the endonucleolytic cleavage kinetics of the DNA of a T4 mutant lacking DNA polymerase, DNA ligase and exonuclease by using the sucrose gradient sedimentation technique. Our results can be summarized as follows. 1. The single-strand endonucleolytic cleavage of the T4 mutant DNA is not a random process. 2. The number of single-strand nicks reaches a plateau level of 10--12 nicks/molecule. 3. The occurrence of a double-strand break is delayed and their number is at any time lower than the number of single-strand nicks. 4. The circular permutation T4 genome, as computer-simulated by the Monte Carlo method, produces a smoothing of the discrete distribution which would be expected if nicks were localized in the promoter sites of late transcription units. We conclude that our findings support the model which relates single-strand DNA nicks to the late transcription initiation sites.