Section 4 — Gene action 



controlled by the episome as shown by transfer 

 experiments. 



When the thermosenisitve F-Lac episome, 

 however, is integrated in the chromosome as 

 Hfr, it replicates normally at 40°, suggesting that 

 in this case F is no longer reproduced by the F 

 system of replication but by the chromosomal 

 one. In bacteria carrying both a thermosensitive 

 F-Lac, and a normal F-Gal factor, the replication 

 of the F-Lac is normal at high temperature, a 

 result which suggests that the thermosensitive 

 factor is diffusible. 



1. F. Jacob and S. Brenner, C.R. Acad. Sci. 

 257, 298-300 1963. 



4.17. The Mechanism of Chromosome Mobilization 

 by F-lac in Escherichia coli K12. John Scaife 

 and Julian D. Gross (London, Great Britain). 



Male cells of E. coli K12 harbour the episomic 

 sex factor, F. During conjugation these are able 

 to donate genetic material to female cells, which 

 do not carry the F-factor. F-episomes incorporat- 

 ing genes from the bacterial chromosome have 

 been isolated, e.g. F-lac. 



The chromosome of E. coli is circular; in 

 conjugation, however, it is transferred as a linear 

 structure. The opening of the circular chromo- 

 some and its subsequent transfer are associated 

 with an interaction between the sex factor and 

 the chromosome. 



In the case of a cell harbouring F-lac this inter- 

 action occurs in the vicinity of the lac region of 

 the chromosome, which suggests that the two 

 structures synapse and interact within the region 

 for which they are genetically homologous. It is 

 proposed that the interaction is, in fact, a genetic 

 exchange in the homologous region, causing the 

 insertion of the F-lac into the chromosome, which 

 then acquires the ability to be transferred as a 

 linear structure. A prediction of this model is 

 that lac will be transferred as the first chromo- 

 somal marker. 



We have made crosses using male cells bearing 

 different lac markers on the chromosome and the 

 F-lac factor. Analysis of the progeny from crosses 

 of this type has yielded results which are con- 

 sistent with this prediction from the model for 

 chromosome mobilization outlined above. 



4.18. High Frequency Resistance Transfer System in 

 Escherichia coli. Tsutomu Watanabe (Tokyo, 

 Japan). 



Cells of Escherichia coli K-12. which have just 



received R factors (episomic resistance factors), 

 are able to transfer them in unusually high fre- 

 quencies unlike usual R + cells. HFRT popula- 

 tions which contain high ratios of such competent 

 donor cells can be obtained following the pro- 

 cedure for obtaining HFCT (or HFC) of Ozeki 

 and Stocker (1960). We have recently found that 

 HFRT contains a large number of cells with 

 higher drug resistance than usual R+ cells. The 

 R factors of the clones which grew on higher 

 drug concentrations were found to have mutated 

 resistance markers which confer higher drug 

 resistance. Jacob et al. (1960) have shown that 

 partial polyploids carrying multiple copies of 

 FMac synthesize unusually large amounts of 

 (3-galactosidase. In view of their finding, our 

 results may be interpreted as due to a gene 

 dosage effect by the unusually vigorous repli- 

 cation of R factors in HFRT cells. Transfer of R 

 factors is very frequent because of the many 

 copies of R factors in cytoplasm and also of the 

 increased production of RTF-specific mating 

 substance. The gene dosage effect is also exerted 

 on the biochemical mechanisms of drug resis- 

 tance and HFRT cells are thus phenotypically 

 more resistant than usual R + cells. HFRT cells 

 are able to divide for several divisions on higher 

 drug concentrations until the R factors are 

 reduced to "stable autonomous state" or in- 

 tegrated state. Accordingly, the probability of 

 mutation of R factors per cell is higher in HFRT 

 cells than in usual R+ cells. 



4.19. Linkage of Colicinogenic Factors with an F 

 Agent and with Nutritional Markers in the 

 Chromosome and in an Episome of Escherichia 

 coli. Pierre Fredericq (Liege, Belgium). 



Some wild strains E. coli producing colicins 

 V or B have been found to carry their colicino- 

 genic factors closely linked to an F agent of 

 sexual polarity. Almost every recombinant ob- 

 tained in crosses with F- derivatives of E.coli 

 K12are F+ or F- according to whether it receives 

 or not the colicinogenic factor. In such crosses, an 

 unusual type of recombinant has been obtained, 

 where the F agent has been integrated in the 

 chromosome. Genetical analysis of that recom- 

 binant revealed that a B colicinogenic factor is 

 still linked with the F agent and integrated with 

 it in the chromosome. In that Hfr strain the 

 leading markers is mal and the most distal str. 

 There is no transfer of the Hfr and colicinogenic 

 properties when selection is made for transfer 

 of the proximal marker met. There is however a 

 linked transfer of both properties when selection 



42 



