Bacteriophage T4 mutants defective in gene 56 (
dCTPase) synthesize
DNA where
cytosine (Cyt) partially or completely replaces hydroxymethylcytosine (HmCyt). This Cyt-
DNA is degraded in vivo by T4
endonucleases II and IV, and by the
exonuclease coded or controlled by genes 46 and 47.-Our results demonstrate that
T4 endonuclease II is the principal
enzyme initiating degradation of T4 Cyt-
DNA. The activity of
endonuclease IV, but not that of
endonuclease II, was stimulated in the presence of a wild-type
dCMP hydroxymethylase, also when no HmCyt was incorporated into phage
DNA, suggesting the possibility of direct
endonuclease IV-
dCMP hydroxymethylase interactions.
Endonuclease II activity, on the other hand, was almost completely inhibited in the presence of very small amounts of HmCyt (3-9% of total Cyt + HmCyt) in the
DNA. Possible mechanisms for this inhibition are discussed.-The E. coli
RNA polymerase modified by the products of T4 genes 33 and 55 was capable of initiating
DNA synthesis on a Cyt-
DNA template, although it probably cannot do so on an HmCyt template. In the presence of an active
endonuclease IV, Cyt-
DNA synthesis was arrested 10-30 min after
infection, probably due to damage to the template. Cyt-
DNA synthesis dependent on the unmodified (33-55-)
RNA polymerase was less sensitive to
endonuclease IV action.