Section 4 — Gene Action 



is made for more distal markers. The rate of 

 transfer of both properties increases from 0.01 

 when selection is made for transfer of pro to 0.70 

 when selection is made for transfer of str, the 

 most distal marker. 



Linkage of colicinogenic factors with nutritio- 

 nal markers has also been observed in an F 1 

 episome. In a cross of a wild strain E. coli, 

 carrying V and B colicinogenic factors linked to 

 an F agent, with a deletion mutant tryABDC, 

 Tl-r of E.coli B, some recombinants, selected 

 for transfer of try + , had a very peculiar behaviour 

 towards phage Tl, being partially susceptible. 

 They were found to be partial diploids, carrying 

 the wild try region, including cysB and 77, 

 linked to the F agent and to the V and B colici- 

 nogenic factors in an F 1 episome. These recom- 

 binants have been further crossed with different 

 cysB or try mutants of E.coli B or K12. The new 

 recombinants appear to carry, like the male 

 parents, an episome in which F is linked to the 

 V and B colicinogenic factors and to the entire 

 cys try region. Preliminary transduction experi- 

 ments by phage Pike grown on such a diploid 

 strain reveal that genetic material from either 

 the chromosome or the episome can be trans- 

 ferred. In many transductants the introduced 

 fragment is not integrated and remains in the 

 episomic state, giving rise to stable diploids. 

 Among them a few received all the known 

 markers from the episome, including the colici- 

 nogenic factors. From the data, the order (F), 

 cysB, tryA, tryB, tryD, tryC, Tl, (colV), (colB) 

 can be inferred. 



The research reported in this document has 

 been made possible through the support and 

 sponsorship of the U.S. Department of Army, 

 through its European Research Office, Univer- 

 sity of Liege, Belgium. 



4.20. Controlling Episomes. G. W. P. Dawson 

 (Dublin, Great Britain). 



Instability at a number of loci in Salmonella 

 typhiimtrium have been studied and their in- 

 stability appears to arise by an attachment of 

 genetic particles to the locus. To point the simi- 

 larities between these particles and controlling 

 elements in higher organisms and episomes in 

 bacteria they are called controlling episomes. At 

 some loci controlling episomes suppress gene 

 expression as well as causing instability. Different 

 loci have different avidities for particular con- 

 trolling episomes. Different strains have different 

 frequencies of controlling episomes. Controlling 



episomes can transpose from one locus to another 



4.21. Controlling Episomes at the su-leuA locus of 

 Salmonella typhimurium. P. F. Smith-Keary 

 (Dublin, Great Britain. 



Slow growing reversions of the leucine re- 

 quiring strain leu-151 of Salmonella typhimurium 

 are due to mutations at one of a number of sites 

 within a linked suppressor locus su-leuA; many 

 of these reversions are highly unstable. This in- 

 stability is caused by a particular type of episome 

 called a controlling episome which becomes trans- 

 posed from some unknown location to a site 

 within su-leuA where it induces a high frequency 

 of mutation. The instability is retained at this 

 site, through successive back and forward 

 mutations of su-leuA, until the controlling epi- 

 some becomes transposed to another location. 



Some strains of leu-151 revert to the unstable 

 types at a much higher frequency than other 

 strains. This is attributed to these strains having 

 a higher frequency of controlling episomes 

 available for transposition to su-leuA; in these 

 strains the frequency of unstable reversions can 

 be greatly reduced by treatment with acridine 

 orange. The frequency of stable reversions is the 

 same in all strains of leu-151 and is unaffected 

 by acridine orange treatment. 



A controlling episome located within su-leuA 

 can be transposed to a locus modifying proline 

 requirement, resulting in instability for proline 

 independence and concomitant stability at 

 su-leuA . 



4.22. Transposed Inducer Function in E. coli. K. C. 



Atwood (Urbana, U.S.A.). 



From a K-12 stock carrying F-lac, a mutant 

 was screened that requires an inducer of B- 

 galactosidase for growth on medium containing 

 glycerol as sole carbon source. Inducing levels of 

 the nonutilizable inducers, methyl-thiogalacto- 

 side or isopropyl-thiogalactoside satisfy the re- 

 quirement. The inducer requirement is also pre- 

 sent with amino acid mixtures or succinate as 

 carbon sources, but not with any hexose or 

 pentose that can be used by the parent strain. 

 Neither induced nor uninduced cells of the 

 mutant produce B-galactosidase. The mutant is 

 interpreted as a transposition of controlling 

 elements such that the system normally in control 

 of B-galactosidase synthesis now controls a 

 different enzyme, apparently an enzyme involved 



43 



