Hydroxyurea-sensitive mutants of T4

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.