Evidence fora NewEndonuclease Synthesized byx Bacteriophage

Infection ofnonlysogenic Escherichia coli CR34(S)(Thy-) withbacteriophage X CI857resulted intheformation oftwisted circular double-stranded phagedeoxyribonucleic acid(DNA;species I).Whensuchinfected bacteria were incubated in theabsence ofthymine, there was a significant decrease intheamountofspecies IDNA after 60minofincubation. A similar loss ofspecies IXDNA during incubationina thymine-deficient mediumwas alsoobserved after infection oftheendonuclease I-deficient strain, E.coli1100(S) (Thy-). Thisdestruction oftwisted, circular XDNA inthymine-deprived cells didnotoccurinthepresenceofchloramphenicol norinlysogenic E.coli CR34carrying a noninducible Xprophage. Itis therefore concluded thattheendonuclease whichattacks this circular configuration ofXDNA isnewlysynthesized after infection andisdirected bythephagechromosome. WhenXbacteriophage infects abacterial cell, a large proportion oftheentering linear duplex deoxyribonucleic acid(DNA)molecules areconverted toa nonended circular form(1,16,17). Thisformofphage DNA corresponds tospecies Iasdescribed byBodeandKaiser (1) andcanbe distinguished byitssedimentation properties in sucrose gradients. Atneutral pH,species Isediments1.9times faster thanlinear XDNA (species III), whereas atalkaline pHitsediments almost fourtimes asrapidly asspecies III. Inneutral sucrose, athird formofXDNA canbeseparated; this formhasasedimentation rateof1.14times thatoflinear XDNA.Itisdesignated species II andisbelieved toconsist ofdouble-stranded circular DNA containing one or more singlestrand breaks. Centrifugation ofspecies IIin alkaline sucrose causesdenaturation oftheDNA which thensediments atnearly thesamerateas species III. Thus,centrifugation inalkaline sucroseclearly separates species Ifromspecies II andIIIbutusually doesnotpermit thedistinction between species IIandIII. Inacontinuing study oftheroleofcircular phage DNA inXdevelopment, wehaveexamined theformation oftwisted circular XDNA (species I)after infection ofthymine auxotrophs inthe presence andabsence ofthymine. Asexpected, theinfection ofthymine-requirin g