Bacteria: Recombination {IV) 



369 



frequency (/io) than, the original Hfr hne. 

 This is exactly what is found in compari- 

 sons of P4x-1 with P4x. 

 5, The F^-Lac+ element can be transmitted 

 in a series of successive conjugations, each 

 recipient possessing the properties of the 

 original recombinant. 



All these results are most simply explained 

 by the deintegration, in the original Hfr 

 strain, of an F^ particle carrying a chromo- 

 somal piece bearing Lac+. The attached Lac+ 

 piece is known, moreover, to contain three 

 cistrons; these govern the synthesis of 13- 

 galactosidase, j8-galactoside permease, and 

 the repressor for this system, respectively. 

 From subsequent integrations and deintegra- 

 tions it is possible also to obtain F'-Lac^ 

 particles, and other particles differing in Lac 

 and F^ capacity. Finally, another Hfr, where 

 F^ is integrated close to Pro, has been found 

 to produce an ¥^-Pro particle having proper- 

 ties analogous to those of the F^-Lac particle. 



Since F' can integrate at a variety of loci, 

 we can extrapolate from such results that as 

 a consequence of deintegration, any one of a 

 variety of normal chromosomal loci may 

 become a part of the genotype of cytoplasmic 

 F^ When long enough, this memory piece 

 of chromosome attached to F' will both repli- 

 cate and produce its phenotypic effect. The 

 evidence seems conclusive, then, that chro- 

 mosomal DNA need not be located in the 

 chromosome in order to replicate or function. 



We see, therefore, that F^ is involved in four 

 types of genetic recombination. F^ makes 

 possible recombination between a chromo- 

 some segment of a donor and the chromo- 

 some of a recipient; F^ can itself be added 

 as a locus to the chromosome at a variety of 

 positions, this being repressive of F^ located 

 extrachromosomally; F^ can be removed 

 from its chromosomal locus in toto (or in 

 part) by deintegration; the deintegration that 

 hberates F^ can include also neighboring 

 cistrons. The last event produces recombina- 



tion of chromosomal genes between F^ and 

 the chromosome. There is, therefore, a 

 two-directional gene flow between the extra- 

 chromosomal episome F and the chromo- 

 some. 



So far, we have restricted our attention 

 exclusively to the episome F in Escherichia 

 coli. Evidence also has been obtained that 

 colicines (certain substances - having the 

 capacity to kill colon bacilli, i.e., having 

 colicidal properties) are produced by a coli- 

 cinogenic determinant which acts as an epi- 

 some in E. coli. The colicinogenic determi- 

 nant can leave its chromosomal locus, multi- 

 ply autonomously extrachromosomally, and 

 later integrate into the chromosome.^ When 

 extrachromosomal, the free episome can mi- 

 grate actively, as F does, arriving in the F~ 

 cell as early as 2}^ minutes after conjugation 

 is initiated. 



Do episomes occur in organisms other than 

 bacteria? It has already been suggested (re- 

 fer to p. 360) that certain genetic elements in 

 the chromosomes of corn and of Drosophila 

 possess characteristics similar to those of 

 episomes.'' In this connection let us also 

 consider the relationship between the centro- 

 mere and centrosome. The centrosome is 

 an organelle often found at each pole of a 

 spindle, particularly in animal cells, and a 

 granular structure called the centriole is some- 

 times seen within it. Similar granules can 

 sometimes be seen within the centromere 

 (Figure 40-1). The granules in both the 

 centromere and centrosome stain the same 

 (both doubtless containing DNA), and so 

 does the material surrounding these granules. 

 In the hving cell, also, centromere and 

 centrosome have a similar appearance. (The 

 granules within the centromere are thicken- 

 ings of the DNA thread which passes through 

 the centromere and which is continuous with 



' At least two colicines are lipocarbohydrate-protein 



complexes. 



3 See F. Jacob and E. L. Wollman (1958). 



■• See F. Jacob and E. L. Wollman (1961), p. 364. 



