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Draft Letter to Nature

Bacterial Sex Factors, Conjugons, and Plasmids

The original bacterial sex factor, the F agent of E. colt K12,

was discovered because it mediates the transfer of host genes to a
recipient cell. Later the term episome was introduced to designate

the broader class of units, including both sex factors and temperate
bacteriophages, that can replicate either autonomously or as an
integrated part of the host chromosome. Since then the plasmid has
emerged as an even more fundamental concept: a bloc of DNA, considerably
smaller than the chromosome, that is capable of autonomous replication.
Members of this class differ from each other, and also can vary in
different hosts, in their secondary properties. Thus some are episomal;
some can cause formation of a virion that can effect their own trans-
mission and can occasionally transduce host genes; some can cause the
formation of a conjugation apparatus that effects their transmission;
some of this last class can, but others cannot, mobilize the host
chromosome for such transmission; and some minute plasmids may lack all
these properties.

Communication in this field is impaired by the lack of convenient
terms to differentiate plasmids that cause conjugation from the others,
and also to differentiate the two separable functions of conjugation
and chromosome mobilization. '"Transmissible plasmid" and "non-transmissible
plasmid" are unsatisfactory, for both kinds are transmissible by
transduction; "“autotransmissible plasmid" is too clumsy; and a sex factor
that does not transfer host genes hardly fits the evolutionary role of
sex. We are therefore proposing to use the term "conjugon" for plasmids
that can generate a conjugation apparatus, just as a virus is defined in
terms of its capacity to generate a virion. Since rates of integration
and conjugation vary widely the term "sex factor" might best be used not as a
characteristic for classifying plasmids but only under circumstances where
mobilization and transfer of the host chromosome is the focus of the
discussion. .

The term "conjugon" has been used in the past (1), but without wide
acceptance. However, the growing need for such a term seems to justify its
revival.

Yours,

1. Luria, S. E. Recent Progr. Microbiol. 8, 604 (1962).