A gene of bacteriophage T4 whose product prevents true late transcription on cytosine-containing T4 DNA.

T-even coliphages have 5-hydroxymethylcytosine in their DNA instead of cytosine. In some T4 mutants, the replicated DNA contains cytosine, but then no late gene products are made. We show that the inability to make late gene products with cytosine-containing T4 DNA is due to a T4 gene products. This gene product, while probably nonessential under normal conditions, interacts with an essential part of the transcription apparatus. Mutations in this gene allow viable T4 particles to be made whose DNA has been substituted almost 100% with cytosine.