356 



« ii \r iir 27 



mum of its recombinants for Pro 01 ll be 



haved us males. 



4. The male factor was linked neither to 

 the above-mentioned loci nor to an) other 

 chromosomal marker showing recombination. 



.v Like tree F, the male factor entered 

 the F cell about five minutes after con- 

 jugation began. 



6. Treatment with acridine orange elimi- 

 nated the male sex factor, converting the 

 cells to F - . 



These findings prove that the P4x-1 male sex 

 factor — called F' — is located extrachromo- 

 somally. 



Although F could attach at any one of a 

 number of different chromosomal sites pro- 

 ducing Hfr chromosomes differing in O point 

 position and in direction of transfer. F' at- 

 tached at a particular locus near Lac in such 

 a way that chromosomal loci were always 

 transferred in the same order and direction. 



Since P4x-1 transferred its chromosome 

 more frequently than the typical F+ (F- 

 containing) male, the chance of F' associat- 

 ing with the chromosome near Lac seems to 

 be greater than the total chance of F integrat- 

 ing at any one of numerous different loci. 

 P4x-1. on the other hand, transferred the 

 chromosome less frequently than P4x, sug- 

 gesting the possibility that F is not fully inte- 

 grated in P4x-1 ordinarily, although it is in 

 P4x. This difference would also explain why 

 P4x-1 had free F\ whereas P4x did not. 

 since chromosomal F prevented the estab- 

 lishment of free F. Ordinarily, F' seems to 

 exist as a kind of exogenote which synapses 

 with the chromosome just before or after 

 conjugation is initiated. The mechanism of 

 chromosome mobilization seems to involve a 

 recombinational event between the episomc 

 F' and the chromosome. 



When F' was transferred to F~ cells as an 

 extrachromosomal particle, the recipient cells 

 were converted to males that, relativelv 



often, could transfer their chromosome in 

 the same sequence as P4.\ and P4.\-l males, 

 suggesting that the chromosome o[ the ordi- 

 nary F cell has a segment of DNA near Lac 

 homologous to a segment carried by F'; that 

 is. F' possesses a chromosomal segment 

 which can pair with a particular chromo- 

 somal locus. 



As mentioned, treatment with an acridine 

 dye eliminated the extrachromosomal F' par- 

 ticles from the P4x-1 strain and converted it 

 to F — . Such an F strain conjugates with 

 males carrying either F or F' extrachromo- 

 somally. In both cases the F strain was 

 relatively often converted into a donor 

 (male) that transferred its chromosome in 

 the same sequence as P4x and P4x-1 males. 

 Clearly, then, the F~ — derived from P4x-1 

 via acridine orange — carries a chromosome 

 which has retained a segment of F near Lac, 

 the portion retained held in common by F 

 and F'. In so far as the F portion of the 

 particle is concerned, then, F and F' were 

 not detectably different in these experiments. 

 Since F' is found to be approximately twice 

 the size of F, one can think of F' as being an 

 F particle with an extra, particular piece of 

 chromosome attached. 



The preceding suggests that F' can carry 

 chromosomal DNA apparently still capable 

 of replication in its new location. Let us 

 suppose that this chromosomal segment is 

 also still able to function normally. F' may 

 fail to show a phenotypic effect for a normal 

 chromosomal locus because it contains one 

 or more (as yet unidentified) chromosomal 

 markers. (Probably less than 1% of the 

 chromosomal loci have been identified; more- 

 over, there may be chromosomal regions 

 whose only function is the maintenance of 

 and recombination with episomes.) The 

 very existence of F', however, encourages a 

 search for still different F particles to which 

 a known chromosomal marker might be 

 attached. 



