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Phase variation

phase variation
(Science: microbiology) alteration in the expression of surface antigens by bacteria.
For example: salmonella can express either of two forms of flagellin, H1 and H2, that are coded by different genes. Control of which form is expressed is brought about by inversion of the promoter for the H2 gene, which if functional (noninverted) is associated with the expression of H2 and the production of a repressor of the H1 gene.
inversion occurs about every 1000 bacterial divisions and is under the control of another gene, hin, that is within the invertable sequence.

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