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A fla mutant of E. coli K12 was given fla  + and H1-i by phage P1kc cotransduction from S. typhimurium, then made Fla- by transduction of ah1 from S. typhimurium. Motile clones expressing a Salmonella phase-2 antigen, e,n,x or 1,2, were obtained from the K12 i  ah1 (therefore Fla-) line by P1kc transduction of flagellin-specifying genes, H2-e,n,x or H2–1,2, from Salmonella donors. Of eighteen such transductants sixteen failed to show phase variation, and on transduction back to Salmonella each structural gene for a phase-2 flagellin (or at least for its antigenically determinant part) now behaved as an allele of H1, presumably in consequence of incorporation in the hag region of the K12 recipient, in place of H1-i ah1 . The e,n,x- and 1,2-specifying genes were shown to have been integrated in the K12 chromosome without the linked H1-repressor gene or the adjacent vh2 gene (controlling rate of phase-variation) and they responded to the repressing activity of an H2 allele elsewhere in the cell, in this respect resembling H1 alleles of Salmonella or hag alleles of E. coli. Two K12 e,n,x transductants had flagellin-specifying genes which when transduced back to Salmonella were integrated at H2; they are inferred to have resulted from integration of H2-e,n,x in the K12 chromosome elsewhere than the hag region. These two clones showed phase variation, between a Fla+ phase, with antigen e,n,x, and a Fla- phase (with e,n,x determinant in the nonactive state and the determinant of antigen i inactivated by ah1). The two integrated e,n,x genes when in the "active" state retained the ability to repress expression of exogenote H1 alleles, which indicates that the closely linked H1-repressor gene also was integrated. One of the two exceptional transductants derived its e,n,x gene from a Salmonella donor with the linked vh2  - gene, which in Salmonella almost entirely prevents change of phase, and transduction of this e,n,x gene back to Salmonella recipients proved that vh2  - had been incorporated into the E. coli chromosome along with the e,n,x determinant and the H1-repressor gene. The high frequency of change of phase (Fla+↔Fla-) in the K12 e,n,x  vh2  - transductant concerned suggests that vh2  - fails to prevent frequent change of state of the phase-determinant part of H2 when vh2  - and H2 are incorporated in the E. coli chromosome.

INTEGRATION, AT HAG OR ELSEWHERE, OF H2 (PHASE-2 FLAGELLIN) GENES TRANSDUCED FROM SALMONELLA TO ESCHERICHIA COLI