Summary: Mutants showing oscillatory variation between H-type carrying phase-1 antigen and O-type were isolated from a diphasic derivative of The rates of variation from the H-type to O-type were about 10/bacterial division and those in the reverse direction 10 to 10, which are very similar to the rates of variation from the phase-1 to phase-2 of the parent strain and vice versa. Transduction analyses showed that their mutation sites were in (the phase-2 flagellar antigen gene). In O-type bacteria the expression of both the endogenote (the phase-1 flagellar antigen gene) and the exogenote introduced by abortive transduction were repressed. These results indicate that the bacteria are flagellate (H-type) in phase-1 and non-flagellate (O-type) in phase-2, i.e. they are -repressor-positive phase-2 non-flagellate mutants (H-O variants). The occurrence of H-O variants strongly supports the hypothesis that there is a special -repressor gene in the operon which is concerned with the regulation of expression in phase-2 bacteria.


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