Bacteriophage T4 alc gene product: general inhibitor of transcription from cytosine-containing DNA.

The alc gene of bacteriophage T4 was originally defined on the basis of mutations which allow late protein synthesis directed by T4 DNA containing cytosine rather than hydroxymethylcytosine. The question remained whether the normal alc gene product (gpalc) also blocks the transcription of early genes from cytosine-containing DNA. Complementation experiments were performed between hydroxymethylcytosine-containing phage which direct gpalc synthesis but carry mutations in a given gene(s) and cytosine-containing phage carrying that gene(s). The required protein would then have to be directed by the cytosine-containing DNA: it is looked for directly on polyacrylamide gels or through its physiological effects or both. For all early proteins examined in this way, no synthesis was observed when 95 to 100% of the hydroxymethylcytosine was substituted by cytosine in the infecting DNA, whereas there was significant synthesis with 75% substitution or less. The results indicate that gpalc is carried in with the infecting DNA or is made very early to block transcription of all cytosine-containing DNA.