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7 HIROTA, Y., Genetics Department, Stanford University, Stanford,

Californie: ts of F_factor righ 1p K-22. -- A

contaglous plasmid F which occurs In the wild type strain K-l2 confers

the capacity to ect as o” In sexual conjugation on strains of Escherichta

coll (Lederberg, Cavalli, and Lederberg). Recently, Adelberg and Burns

(these abstracts) found a mutant of F termed F' which conveys the

Hfr character (High frequency of recombination). On the basis of this

finding, we have Isolated distinctive F' mutants from old broth cultures

of several Hfr strains. The F' sutants were detected by the conversion

of F- strains with which they were Incubated. Each F' mutant controls

a characteristic pattern of transfer of loci from the o' chromosome in

the linkage sequence similar to that of the original Kfr from which

was derived. Like stendard F+ the new F' can be disinfected by acridine

treatment (Hirota and tijima) and passed to other F- strains. ---- These

results suggested that the F' factor fis split off from the Hfr locus

and llberated Into the cytoplasm. Although free of the chromosomal

site, It still determines transfer of the chromosome with high frequency, .

the potnt of chromosome breakage, and controls the sequence of oriented

transfer of genes of the donor chromosome during conjugation. its

specificity might be from the retention of a chromosome segment of a

chromosome segment of the Hfr parent from which the F' orlginates.

Double infection as well es mutual exclusion have been observed In

combination of nonidentical F factors.
Added note: XKX  F' is used [In this abstract as a class term for F mutants
of which F® (Adelberg and Burns) is an example. F' (F-prime) should not be

confused with Fl (F-one) as they use to denote the wild type F factor. The

notation is belng rectified,