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Phase Variation of Flagellar Antigens in Salmonella: Abortive Transduction Studies
Abstract
SUMMARY
Motility was transduced by phage P22 to non-motile (fla –, etc.) Salmonella typhimurium 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 fla – gene closely linked to H1 (phase-1 flagellar antigen locus) nearly all the fla + 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 H1 allele, like that of the chromosomal H1 allele, was regulated by the phase-determinant of the recipient, at or near H2 (phase-2 flagellar antigen locus). An H1 gene adjacent (cis) to ah1 – (H1 activator gene), in the chromosome or in the exogenote, was unexpressed in ah1 + or fla + abortive transductants in phase 1. This suggests that ah1 – mutants are H1-operator-negative mutants.
When the recipient was a phase-1, and therefore non-motile, culture of an ah1 – or ‘phase-1-curly’ mutant, lysates of phase-2 cultures, but not of phase-1 cultures of the same donor, evoked trails, attributable to H2 abortive transductants. They expressed the donor H2 allele, but not the recipient H2 allele—nor the previously expressed H1 allele of the ‘phase-1-curly’ recipient. It is inferred that a phase-determinant regulates the expression of the H2 gene adjacent (cis) to it (or of which it forms part) but not that of another H2 gene in the same cell; and that it controls the expression of H1 via a represser substance, not via an inducer. The exceptional H1 allele H1-1,2 determines a flagellin of antigenic character 1,2, apparently identical with that determined by a common H2 allele. H1-1,2 and the common H1 allele H1-b were simultaneously expressed in fla + abortive transductants, which suggests that neither H2 flagellin nor H2 messenger RNA functions as the repressor of H1. It is proposed that an operon, comprising H2 and the structural gene for an H1 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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