Section 3 — Molecular and Microbial Genetics 



result from more than one factor and cannot 

 be ascribed to one gene with a pleiotropic 

 effect. 



1. P.N.A.S. 1956. 



3.23. Mutants of Prophage PI which Affect Host- 

 induced Modification. S. W. Glover (London, 

 Great Britain). 



Bacteriophage A plates with an efficiency of 

 1 .0 on Escherichia co/i strain C 600 (K), and with 

 an efficiency of 10 4 on K made lysogenic for 

 phage PI. This phenomenon is called restriction. 



The A phage which comes through K(P1) plates 

 with an efficiency of 1.0 on both K and K(P1) 

 and is designated A (PI). This is called modifi- 

 cation. After one cycle of growth in K the 

 modified phage A(P1) loses its modification. 



By several different methods to be described 

 derivatives of K(P1) have been isolated in which 

 the carried PI prophage is abnormal. So far two 

 types of abnormal PI have been characterized. 

 One type has lost the ability to restrict A but is 

 still able to modify it. The other type has lost 

 both the restricting and modifying functions of 

 PI. The results of genetic crosses between these 

 abnormal PI phages will be discussed. 



3 .24. Different DNA Systems undergoing Host- 

 induced Modification in Bacteria. J. Schell 

 and S. W. Glover (London, Great Britain). 



In the system of host-induced modification by 

 bacteria carrying prophage PI, A DNA is 

 restricted and modified (see previous abstract, 

 S. W. Glover). 



Data will be presented and discussed concern- 

 ing the restriction and modification of DNA from 

 other sources, including: 



(a) Chromosomal DNA in conjugation. 



(b) Episomal DNA in Flac infection. 



(c) Colicin factors in colicinogeny transfer. 



(d) Phages other than A. 



The nature of the abnormal PI prophage 

 described in the previous abstract (S. W. Glover) 

 has been investigated with these systems. 



3.25. Affinity of Coliphage P2 for Prophage Sites. 

 Erich W. Six (Iowa City, U.S.A.). 



Affinity of P2 for its prophage sites (I, II, etc.) 



on the chromosome of Escherichia coli strain C 

 was studied. Usually strain C derivatives with 

 P2 at site I liberate phage with strong preference 

 for this site, whereas phage from donors car- 

 rying P2 at site II show no strong site preferences. 



The site occupied in the donor cannot be 

 solely responsible for the site affinity of the pha- 

 ges: P2 may retain its low site I affinity even 

 while occupying this site in the donor, and the 

 site I affinity of phages from donors carrying 

 P2 at site II or other sites -/- I varies. 



P2 from donors producing phage with strong 

 preferance for site I occasionally looses this prefer- 

 ence when lysogenizing another cell. Donors 

 producing phage with low site I affinity also pro- 

 duce some phages with strong site I preference. 



All P2-like phages, including P2 Hy dis and 

 PK, show some affinity for site I, even if liber- 

 ated from donors other than E. coli strain C. 

 But P2-related phage P4 shows no affinity for 

 site I. 



Existence of P2 in at least two states: one 

 with high preference for a particular chromoso- 

 mal site, one without particular site preference, 

 resembles the behavior of fertility factor F. 

 Since only relative affinities were measured, 

 loss of site I preference may indicate either 

 decrease of affinity for this site or increase in 

 affinity for other sites. 



3.26. On the Induction of Lysogenic Bacteria by 

 Halogenated Pyrimidines. E. Geissler (Berlin, 

 Germany). 



The paper summarizes results of experiments 

 on the induction of Escherichia coli K12 (lamb- 

 da) by halogenated pyrimidines. An induction 

 results, if the lysogenic bacteria are incubated in 

 SS-medium (containing Bacto vitamin-free 

 casamino acids) supplemented with 0.5 ug/ml 

 of either 5-fluorouracil (FU) or 5-fiuorodeoxyu- 

 ridine (FUdR). 



The inducing activity of FUdR is greatly 

 enhanced by 200 ug/ml 5-bromouracil (BU) — 

 which itself does not induce — even if an ap- 

 parently sublethal concentration of FUdR 

 (0.05 ug/ml) is used. BU is ineffective in combina- 

 tion with a further reduced concentration of 

 FUdR (0.005 ug/ml), or if its own concentration 

 is lowered (20 ug/ml). 



The inducing activity of FU seems to be non- 

 specific with regard to the later stages of nu- 

 cleic acid synthesis: The effect can be reversed 

 by 200 ug/ml BU, 0.5 ug/ml uracil, 25 ug/ml 

 deoxycytidine (CdR) or 200 ug/ml thymidine. 

 These substances revert the bacteriocidic ac- 

 tion of FU too, but a nearly normal multiplica- 



26 



