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CHAPTER 40 



frequently with the same marker sequence 

 found for P4x and P4x-1. This demonstrates 

 that an ordinary F~ cell carries a chromosome 

 which has, near Lac, a segment of DNA 

 which is homologous to a segment carried 

 by F^; that is, F- has a chromosomal segment 

 serving as memory for pairing and integra- 

 tion. 



It is possible, also, by treatment with acri- 

 dine orange, to eliminate the extrachromo- 

 somal F particles from the P4x-1 strain, con- 

 verting it to F^. Such a strain will conjugate 

 with males carrying either F^ or F^ extra- 

 chromosomally. In both cases the F" strain 

 can relatively frequently become a donor 

 (male) which transfers its chromosome with 

 the same orientation as does P4x and P4x-1. 

 Clearly, then, the F~ chromosome derived 

 from P4x-1 has retained near Lac a segment 

 of F- as memory, the portion retained being 

 one held in common by F' and F-. This 

 experiment shows, moreover, that so far as 

 the F portion of the particle is concerned, 

 F' and F- are not detectably different. Ac- 

 cordingly, we can think of F- as being an F^ 

 particle with a longer, particular piece of 

 chromosome attached. 



Since, thus far, the experimental results 

 obtained support our expectations, let us 

 continue our reasoning further. We have 

 found evidence that F may carry chromo- 

 somal DNA which is apparently still capable 

 of replicating in its new location. Let us 

 suppose also that this chromosomal segment 

 is still capable of performing its normal func- 

 tion. The fact that F^ carries no phenotypic 

 effect expected of a normal chromosomal 

 locus was presumed to mean that its chro- 

 mosomal piece is short. Suppose a cistron 

 usually contains hundreds or thousands of 

 linearly arranged deoxyribotides. F' might 

 carry, say, four chromosomal nucleotides as 

 memory. In that case, the piece of chromo- 

 some would contain only part of a cistron 

 and could therefore produce only a portion 

 of the product of the complete cistron. Since 



an incomplete primary gene product would 

 not be unique, F' would be scored in practice 

 as producing no gene product for this cistron. 

 Yet the shortness of its chromosomal piece 

 would mean that there would be many places 

 where F' would find a similar deoxyribotide 

 sequence when placed alongside the chromo- 

 some in one direction or the other. This 

 would explain the fact that F' can integrate 

 at a number of loci, giving rise to Hfr or Vhf 

 lines that transfer markers in opposite se- 

 quences. 



The failure of F- to show a phenotypic 

 effect for a normal chromosomal locus may 

 be due either to the fact that its chromosomal 

 piece, though longer than that of F', is still 

 not long enough to include a cistron, or that 

 it contains one or more, as yet unidentified, 

 chromosomal cistrons. If the latter possibili- 

 ties occur in fact, we should expect to find still 

 different F' particles to which a known chro- 

 mosomal marker is attached. This last ex- 

 pectation can be tested experimentally. 



In studies of interrupted conjugation, using 

 Lac'F~ cells and a Lac^ strain of Hfr where 

 F^ is known to be integrated very close to 

 Lac+, rare recombinants are obtained which 

 have received Lac^ too early. Certain of 

 these recombinants have the following prop- 

 erties: 



1 . They have received only F^ and Lac+. 



2. They are unstable, and occasionally give 

 rise to Lac~¥~ individuals. Hence the 

 original recombinant was a merozygote 

 carrying both Lac+ and Lac~ alleles. 



3. When crossed to Lac~F~ cells they simul- 

 taneously transfer F^ and Lac^ together 

 with 50% or higher frequency. This 

 transfer starts at a time soon after conju- 

 gation begins, just as in the case of free 

 F' (or F-), and is unlinked to other chro- 

 mosomal markers. Thus, F'-Lac+ behaves 

 as a free single unit. 



4. The recombinant transfers its chromosome 

 in the same sequence as, but with a lower 



