Wild-type bacteriophage T4 was enriched for mutants which fail to degrade Escherichia coli deoxyribonucleic acid (DNA) by the following method. E. coli B was labeled in DNA at high specific activity with tritiated thymidine (3H-dT) and infected at low multiplicity with unmutagenized T4D. At 25 min after infection, the culture was lysed and stored. Wild-type T4 degrades the host DNA and incorporates the 3H-dT into the DNA of progeny phage; mutants which fail to degrade the host DNA make unlabeled progeny phage. Wild-type progeny are eventually inactivated by tritium decay; mutants survive. Such mutants were found at a frequency of about 1 % in the survivors. Eight mutants are in a single complementation group called denA located near gene 63. Four of these mutants which were examined in detail leave the bulk of the host DNA in large fragments. All eight mutants exhibit much less than normal T4 endonuclease II activity. The mutants produce somewhat fewer phage and less DNA than does wild-type T4. I This work was perfonned under contract with the U.S. Atomic Energy Commission at the University of Rochester Atomic Energy Project and has been assigned Report No. UR 49-1346. A preliminary report of this work has appeared (Fed. Proc. 29:465, 1970).

SP62 is a mutant of bacteriophage T4D that was discovered because it produces fewer phage than the wild type in the presence of 5-fluorodeoxyuridine. In the absence of phage DNA synthesis, SP62 solubilizes host DNA slower than normal; this may explain the sensitivity to 5-fluorodeoxyuridine. In Escherichia coli B at 37 C in the absence of drugs, SP62 makes DNA at a normal rate and the kinetics of appearance of phage are nearly normal. Under the same conditions, SP62 produces T4 lysozyme (gene e) at a normal rate until 20 min, but then produces it at twice the normal rate until at least 60 min. It has long been known that, when T4 DNA synthesis is blocked (DNAstate) in an otherwise normal infection, the synthesis of a number of early enzymes continues beyond the shutoff time of about 12 min seen in the DNA+ state, but still stops at about 20 min. We have termed the 12-min shutoff event Si and the 20-min shutoff event S2. We show here that, in the DNA+ state, SP62 makes four early enzymes normally, i.e., Si occurs. However, in the DNAstate (where Si is missing), SP62 continues to make dCTPase (gene 56), dCMP hydroxymethylase (gene 42), and deoxynucleotide kinase (gene 1) for at least an hour; this results in production of up to 13 times the normal level of dCTPase at 60 min after infection, or 6 times the DNA-level. We conclude that SP62 is defective in the second shutoff mechanism, S2, for these three enzymes. In contrast, SP62 causes premature cessation of dTMP synthetase production in the DNAstate; the result is a twofold underproduction of dTMP synthetase. Autoradiograms of pulse-labeled proteins separated by slab-gel electrophoresis in the presence of sodium dodecyl sulfate show that a number of other T4 early proteins, including the products of genes 45, 46, and rIIA, are synthesized longer than normal by SP62 in the DNAstate. Few late proteins are made in the DNAstate, but in autoradiograms examining the DNA+ state there is little or no effect of the SP62 mutation on the synthesis of T4 late or early proteins. Circumstantial evidence is presented favoring a role for the gene of SP62 in translation of certain mRNAs. At very high temperatures (above 43 C) in the absence of drugs, phage production, but not DNA synthesis, is much reduced in SP62 infections relative to wild-type T4 infections; this temperature sensitivity is greater on E. coli CR63 than on E. coli B. This property has facilitated recognition of the SP62 genotype and aided in complementation testing and genetic mapping. A later publication will provide evidence that SP62 defines a new T4 gene named regA, which maps between genes 43 and 62. The regulation of RNA and protein synthesis richia coli has been reviewed recently (20, 34, in bacteriophage T4-infected cells of Esche-36, 38, 60); regulatory phenomena have been 'Present address: Dept. of Biology, Yale University, New shown to operate at the levels of both transcrip-Haven, Conn. 06D20. tion and translation. Epstein et al.

with Ai), the poloidal flux within the plasma. Host of the calculations reported here have a = 0 = 2, resulting in flat profiles near the magnetic

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