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Mutagenized T4 was screened for hydroxyurea sensitivity on mutant strain Escherichia coli OK305, which was growing slowly on a synthetic medium. Most of the hydroxyurea-sensitive mutants of T4 isolated in this manner are different from those previously described by Warner et al. (1970) and Hercules et al. (1971), and several may be in new cistrons. Most of them produce fewer progeny per infected cell than wild-type T4, even in the absence of hydroxyurea. The first seven mutants studied map at seven distinct sites and affect at least five different cistrons which are scattered throughout the major early region of the T4 genetic map (between gene 38 and rI). At least one of these mutants is defective in the ability to degrade E. coli DNA.

Hydroxyurea (HU) has been used successfully (Warner el al. , 1970; Hercules et al. , 1971) to select bacteriophage T4 mutants which are deficient in degrading Escherichia coli DNA after infection. Using a different set of plating conditions, HU has also been used to isolate another set of T4 mutants ( hus ) ( Goscin and Hall, 1972 ). All the backcrossed hus mutants degrade bacterial DNA normally. The original hus 13 contained another mutation N. The double mutant hus 13-N degrades E. coli DNA slowly in the absence of phage DNA synthesis, leaving DNA fragments with a molecular weight midway between the sizes of fragments left by mutants defective in genes 46 and 47 and by endonuclease II ( den A) mutants at similar times after infection. Endonuclease IV activity is normal in hus 13-N infected cells. The HU sensitivity of three mutants, hus 3 (which is deficient in exonuclease A), hus 7 and hus 23 is partially due to inefficient utilization of host-derived deoxyribonucleotides as substrates for phage DNA synthesis. This is related to the DNA delay phenotype of these 3 mutants. The mutants hus 7 and hus 23 also appear to package phage DNA into phage particles inefficiently. The backcrossed hus 13 is apparently HU sensitive due to a defect in packaging DNA into phage particles. The HU sensitivity of two other mutants ( hus 19 and hus 20) remains unexplained as these mutants degrade bacterial DNA normally, synthesize phage DNA normally using host-derived deoxyribonucleotides, and package the DNA into phage particles efficiently.