Endonucleolytic Cleavage of Parental DNA and T4 Late‐Gene Expression: Distribution Analysis of Single‐Strand and Double‐Strand Breaks

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Abstract
To investigate the dependency of late transcription on concurrent DNA replication during bacteriophage T4 development, the endonucleolytic cleavage kinetics of the DNA of a T4 mutant lacking DNA polymerase, DNA ligase and exonuclease was analyzed by using the sucrose gradient sedimentation technique. The single-strand endonucleolytic cleavage of the T4 mutant DNA is not a random process. The number of single-strand nicks reaches a plateau level of 10-12 nicks/molecule. The occurrence of a double-strand break is delayed and their number is at any time lower than the number of single-strand nicks. The circular permutation of 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. These findings support the model which relates single-strand DNA nicks to the late transcription initiation sites.