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F Factor

	F Factor During conjugation there is transfer of genetic material between two conjugants belonging to sexually different strains. There is mutual recognition of 'male' and 'female' cells. This is achieved by complementary surface molecules, the male component of which is determined by the F (for fertility) factor or the sex factor. The F factor determines sexuality and conjugation in E coli. Depending upon the presence or absence of the F factor, two mating types have been found in the K12 strain of the bacterium. Cells in which the F factor is present are designated as F+ (male or donor cells). Cells in which the F factor is absent are called F – (female or recipient cells)
Some sex factors are plasmids and others are episomes. The plasmids occur as independent rings of DNA in the cytoplasm When a plasmid is capable of being integrated with chromosomal DNA it is called an episome. The two terms are, however, sometimes used synonymously. In E. coli the F+ males usually have only one F plasmid. Replication of this plasmid is synchronized with replication of the chromosome
The following results have been obtained by crossing or growing F+ and F - strains together. (i) F - x F - crosses are always sterile. The F - strain cannot support conjugation when crossed with another F - strain. (ii) F+ x F - crosses are fertile, but conjugation is a one-in-a-million rarity. (iii) F+ x F + crosses are also fertile, but at a very low level. The fertility can be explained by the fact that some F + cells are converted into F - genotypes, and also by the production of F phenocopies (cells which are phenotypically but not genotypically F -)
(iv)When F + and F - strains are grown together, upto 70% of the F - cells are converted into F + by cell contact. The F + cells transfer their P factors to the F - cells without much chromosomal transfer. The F + cells, however, remain P + (infectious transfer by conjugation). (v) Prolonged incubation with acridine dyes, which are mutagens, converts F+ strains into F - (vi) Certain P+ subs trains show a xl,000 increase in the rate of recombination with F- strains (P + x F -), and are called Hfr (High frequency of recombination) strains. The Hfr strain is produced by the integration of the F factor within the bacterial chromosome.

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