/r ( ) was infected, at 30 °C, with T4D , T4D ( , , DNA-negative phenotype), and T4D ( , , maturation-defective phenotype). A genetic (‘transformation’) assay was used to monitor transcription of genes (polynucleotide ligase), (deoxycytidylate hydroxymethylase), (DNA polymerase), , and (endolysin). The principal results are: (1) All of the genes studied were transcribed exclusively from the so-called -strand of phage DNA. (2) DNA synthesis and the maturation-defective proteins were required to turn-off transcription of genes , and . Experiments performed with chloramphenicol suggested that all phage-specific proteins required to turn-off transcription of these genes were not present until 6 to 8 min post infection (p.i.). (3) During a normal developmental programme, gene was transcribed throughout the eclipse. DNA-negative and maturation-defective conditions had no obvious effect on transcription of this gene. (4) During a normal lytic event, two discrete waves of gene transcription were observed. The late wave was dependent upon DNA-synthesis and presence of functional maturation-defective proteins. The early wave was unaffected by DNA-negative or maturation-defective conditions. Experiments with chloramphenicol indicated that, if any virus-specific proteins are involved with regulation of early transcription, such proteins are present by 3 min p.i.

The data are interpreted to mean that early gene transcription is regulated by a minimum of two mechanisms. One of these mechanisms is fully operational by the 3rd min and, among the genes studied, controlled early transcription. A second mechanism becomes operational between 6 and 8 min p.i. and controls transcription of genes , and .


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