SUMMARY: Motility was transduced by phage P22 to non-motile (, etc.) strains, and the flagellar antigens of abortive transductants inferred from the inhibition by antisera of the trails they produced in semi-solid medium. When the recipient had a gene closely linked to (phase-1 flagellar antigen locus) nearly all the abortive transductants evoked from a recipient culture in latent phase 1 manifested the phase-1 antigens of both recipient and donor, whereas those from a culture in latent phase 2 showed neither phase-1 antigen. Thus the expression of an exo-genote 1 allele, like that of the chromosomal H 1 allele, was regulated by the phase-determinant of the recipient, at or near H 2 (phase-2 flagellar antigen locus). An H 1 gene adjacent () to ( activator gene), in the chromosome or in the exogenote, was unexpressed in or abortive transductants in phase 1. This suggests that mutants are -operator-negative mutants.

When the recipient was a phase-1, and therefore non-motile, culture of an or ‘phase-1-curly’ mutant, lysates of phase-2 cultures, but not of phase-1 cultures of the same donor, evoked trails, attributable to abortive transductants. They expressed the donor allele, but not the recipient allele—nor the previously expressed allele of the ‘phase-1-curly’ recipient. It is inferred that a phase-determinant regulates the expression of the gene adjacent () to it (or of which it forms part) but not that of another gene in the same cell; and that it controls the expression of via a represser substance, not via an inducer. The exceptional allele determines a flagellin of antigenic character , apparently identical with that determined by a common allele. and the common allele were simultaneously expressed in abortive transductants, which suggests that neither flagellin nor messenger RNA functions as the repressor of It is proposed that an operon, comprising and the structural gene for an repressor substance, has alternative metastable states, ‘on’ in phase 2 and ‘off’ in phase 1, comparable to the wild-type state and to that of an operator-negative mutant.


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