;r>s 



CHAPTER 27 



eration.) Populations of donors, grown to 



saturation density in aerated broth or cul- 

 tured on agar overnight, can lose their donor 

 phenotype temporarily and behave as genetic 

 recipients. Since they retain their sex factor 

 yet behave as F cells, they are known as 

 /•" phenocopies ( see p. 317). If a /^/c~F~ 

 phenocopj carrying F is mated with an F^ 

 male carrying F-Lac + , exconjugants can be 

 obtained that carry both types of F. Soon, 

 however, in some experiments, one or the 

 other F particle persists in the progeny. This 

 adjustment shows that there is some regula- 

 tion of the number of F partieles per nuclear 

 body. Hfr which are F~ phenocopies, do 

 not tolerate the presence of an introduced 

 autonomous sex factor. 



F not only mobilizes the entire chromo- 

 some in Hfr cells, but it also mobilizes 

 merogenotes — in an F-merogenote trans- 

 fer - — as shown in the preceding discussion. 

 This latter process has also been called sex- 

 duction, F-duction, or F-mediated transduc- 

 tion. With respect to such transduction, sub- 

 stituted sex factors resemble Adg, just as F 

 resembles A. One can construct a haploid 

 Hfr containing two attached F factors so 

 integrated that, upon mating, the chromo- 

 some is present in two pieces — 1 4 and -'{. its 

 length — each with an F at the end, and both 

 merogenotes capable of being transferred 

 to the F~ cell. An F-merogenote carrying 

 the markers Pur, V6, and Lac transfers the 

 merogenote so that the entry order — deter- 

 mined by studies of spontaneous and artifi- 

 cial interruptions of mating — is 0-Pur-V6- 

 Lac-F. In all these respects, then, whole 

 chromosome and merogenote mobilization 

 by F appear identical, differing only with 

 regard to the length of the genetic segment 

 transferred. 



Although the merogenote markers can 



-' The remainder of this section is based upon A. J. 

 Clark and E. A. Adelherg (1962). A. M. Camp- 

 hell (1962). and W. Hayes (1964). 



sometimes integrate into the chromosome, 

 F-linked merogenotes can also persist and 

 replicate without integration, forming clones 

 of merozygotes or partial diploids. The 

 longer the merogenote — when it consists of 

 49? or more of the genome — the more un- 

 stable it is. 



Extrachromosomal F can pass from male 

 to female during conjugation. Deintegration 

 and integration of F can result in a two- 

 directional flow of chromosomal genes be- 

 tween F and the chromosome in the same 

 cell. When F is at a chromosomal locus, 

 the chromosomal genes are rendered mobile 

 so that chromosomal genes can be trans- 

 ferred to another cell. F can also enter an 

 F~ cell by transduction, in which case it is 

 recovered only in the free state, even if the 

 donor was Hfr. (During phage growth, 

 fragmentation of the chromosome takes place 

 at or close to the ends of the integrated F 

 element.) Consequently, the F particle is 

 directly involved in genetic recombinations 

 within and between bacteria which involve F 

 itself and chromosomal genes. 



Promoters 



A promoter is a genetic element which pro- 

 vides one or more of the special conditions 

 needed for genetic transfer via conjugation. 

 If a genetic element (like F) performs all the 

 functions of a promoter, including mobiliza- 

 tion of the entire chromosome or of a mero- 

 genote, it is called a sex factor. Sometimes 

 a promoter (such as F) promotes only its 

 own transfer (into F~ cells). At other times 

 a promoter and a whole chromosome or a 

 merogenote are transferred in linkage. 



F-Lac promotes the transfer of both the 

 merogenote and the chromosome. After 

 ultraviolet treatment of F-Lac heterogenotes, 

 some individuals are found no longer able to 

 transfer Lac or chromosomal markers. Ap- 

 parently a mutation in F resulted in a loss 

 of one or more promoter functions. When 



