Section 3 — Molecular and Microbial Genetics 



presumably facilitating the recombination be- 

 tween DNA molecules. 



1. A. W. Kozinski, Virology, 13, 124, 1961. 



2. A. W. Kozinski and P. B. Kozinski, Virology, 

 1963 (in press). 



3.13. The Arrangements of Nucleotide Sequences 

 in T2 and T5 Bacteriophage DNA Molecules. 



C. A. Thomas, Jr. and I. Rubenstein (Balti- 

 more, U.S.A.). 



The genetic map of T2 and T4 bacteriophage is 

 circular but the DNA molecule that is liberated 

 by phenol extraction is a linear duplex of poly- 

 nucleotide chains. If the genetic map is related 

 to the physical structure of the DNA molecule 

 the problem arises as to how a linear molecule 

 can give rise to a circular map. An explanation 

 can be made on the basis that the bacteriophage 

 liberate molecules which have nucleotide se- 

 quences which are circular permutations of each 

 other. Thus, markers which are most distant on 

 one molecule are closest together on another. 

 To test this hypothesis, the middles of T2 and 

 T5 DNA molecules were mechanically deleted 

 and the absence of certain nucleotide sequences 

 was tested by "renaturation" or "reannealing" 

 experiments using columns containing dena- 

 tured DNA immobilized in agar beads. The re- 

 sults indicate that when the middles are deleted 

 from the T5 DNA molecule, some special se- 

 quences are removed; whereas, when the middles 

 are deleted from the T2 DNA molecule, no 

 special group of sequences are removed. This 

 would indicate that the T2 molecule had a 

 random beginning point while the T5 molecule 

 has a special beginning point. These results would 

 predict that T5 should have a single linear ge- 

 netic map. 



3.14. The Action of Streptonigrin on Genetic Re- 

 combination between Bacteriophages. Myron 

 Levine and Marcene Borthwick (Ann 

 Arbor, U.S.A.). 



The antibiotic streptonigrin preferentially af- 

 fects DNA metabolism in bacteria and is an 

 excellent inducer of phage production in indu- 

 cible lysogenic bacteria^ 1 ). Streptonigrin also 

 has a striking effect on phage recombination. 

 Short treatments (as short as 2 min exposures 

 to lOrg/ml of the antibiotic) of mixedly in- 

 fected E. coli cells results in 2-3-fold increases in 

 the frequency of recombination between T4rII 



mutations. Of special interest is the finding of a 

 critical time during which infected complexes 

 must be treated in order to obtain this effect on 

 recombination. Increases in recombination fre- 

 quencies are observed following treatment of 

 infected cells early in the latent period, within 

 the first 10 min. Treatments at later times in the 

 latent period result in normal recombination 

 frequencies. The data suggest a change in sus- 

 ceptibility to the action of streptonigrin on 

 genetic recombination at about the 8th to 10th 

 minute of the infection. Similar effects have been 

 observed with phage P22 of Salmonella typhi- 

 murium. 



1. M. Levine and M. Borthwick, Bacteriologi- 

 cal Proceedings, 1963. 



3.15. Correlated Variation of Recombination Potential 

 and DNA Content among T4 Phage Particles. 



Gisela Mosig (Cold Spring Harbor, U.S.A.). 



A population of T4 phages is heterogeneous 

 with respect to the following properties which 

 appear to be correlated: 



(a) potential for genetic recombinants. 



(b) buoyant density in CsCl 



(c) size of their DNA molecules. 



Phages from prematurely lysed cultures show 

 slightly greater buoyant density in CsCl, and, 

 when used as parents in crosses, yield a lower 

 fraction of recombinants for given markers 

 than phages from spontaneously lysed cultures. 



DNA isolated from unselected phage particles 

 falls into three classes differing in molecular weight 

 (recognized by column chromatography, sedi- 

 mentation rate, and susceptibility to hydrody- 

 namic shear). If unit length is assigned to the 

 majority (class I DNA), class II DNA measures 

 0.6, and class III DNA varies between 0.05 and 

 0.2. The DNA from these classes does not 

 differ in base composition. 



Phages of different buoyant density differ in 

 DNA content. Those of average density (the 

 majority) contain a single molecule of class I 

 DNA. Less dense phages yield many recombi- 

 nants and contain DNA of class II. Dense phages 

 yield few recombinants and contain DNA of 

 classes I and III, or of class III only. A small 

 fraction of very low density phages also contain 

 DNA of class III only. 



The following working hypothesis is proposed: 

 The dense phage particles yield few recombinants 

 because they are diploid over a wide range of 

 their genome and the diploid state interferes with 

 heterologous pairing. 



23 



