Physical studies of lysogeny: II. Some properties of segments of parental λ DNA found on large pieces of DNA in lysogenizing populations of bacteria

Abstract

High molecular weight (150 to 200 million a.m.u.) intracellular bacterial DNA was extracted from sensitive Escherichia coli cells infected with bromodeoxy-uridine-labeled lambda; 70 to 90% of the cells were destined to become lysogenic. The CsCl density-gradient profile of the intracellular DNA indicated that the parental phage label banded in three positions corresponding to heavy-, hybrid- and light-density class of parental λ DNA. The presence of parental DNA banding near the light-density position of the bacterial DNA was explained by an association of a segment of λ DNA with a fragment of light DNA the size of which was relatively large compared to the size of the λ DNA segment. The size and composition of this light-density parental DNA was studied using CsCl and sucrose gradient techniques. This parental phage DNA was composed of at least two size classes: 70% consisted of a large-size class calculated to be in the range 10 to 60 million a.m.u., and 30% consisted of a very small but undetermined size-class. The composition of the large-size class of parental DNA was influenced by the multiplicity of infection. At high multiplicities, incorporation of both parental strands was favored relative to the incorporation of duplexes containing only one parental strand.