Phase Variation of Flagellar Antigens in Salmonella: Abortive Transduction Studies

Abstract

Motility was transduced by phage P 22 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 H 1 (phase-i 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 exogenote H 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 (cis) to ah1- (H 1 activator gene), in the chromosome or in the exogenote, was unexpressed in ahl+ or fla+ abortive transductants in phase 1. This suggests that ahl" mutants are H 1-operator-negative mutants. When the recipient was a phase-1, and therefore non-motile, culture of an ahl" or "phase-l-curly- mutant, lysates of phase-2 cultures, but not of phase-1 cultures of the same donor, evoked trails, attributable to H 2 abortive transductants. They expressed the donor H 2 allele, but not the recipient H2 allele[long dash]nor the previously expressed H 1 allele of the "phase-1-curly" recipient. It is inferred that a phase-determinant regulates the expression of the H 2 gene adjacent (cis) to it (or of which it forms part) but not that of another H 2 gene in the same cell; and that it controls the expression of H 1 via a repressor substance, not via an inducer. The exceptional H 1 allele H 1-1,2 determines a flagellin of antigenic character 1,2, apparently identical with that determined by a common H 2 allele. H 1-1,2 and the common H 1 allele H 1-b were simultaneously expressed in fla+ abortive transductants, which suggests that neither H 2 flagellin nor H 2 messenger RNA functions as the repressor of H 1. It is proposed that an operon, comprising H 2 and the structural gene for an H 1 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.