Regulation of Flagillin Genes:


Salmonella bacteria have genetic potentiality to evade immune attack and immune recognition by changing its surface proteins. Changing motifs in Flagillin proteins does this trick.

         Bacteria contain two Flagillin genes, one called H2 and the other is called H1.

The H2 gene consists of two cistrons, one for H2 Flagillin and the other for repressor protein for H1 Flagillin gene.

         When the H2 gene is active, it produces not only the surface Flagillin, but also produces repressors. Single promoter controls both.

The repressor binds to the operator of the H1 and it is more or less blocked, all the times.

         Occasionally, the promoter segment of the H2 gene undergoes specific recombination, which results in the reorientation of the promoter segment i.e. in opposite orientation.

This prevents the expression of H2 gene, so repressors are not produced.

         As there are no repressors, the H1 gene becomes active and produces a new Flagillin.







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Regulation of flagellin genes in Salmonella: phase variation. The products of genes fliC and fljB are different flagellins. The hin gene encodes the recombinase that catalyzes inversion of the DNA segment containing the fljB promoter and the hin gene. The recombination sites (inverted repeats) are called hix (yellow). (a) In one orientation, fljB is expressed along with a repressor protein (product of the fljA gene) that represses transcription of the fliC gene.


Switching gene expression by DNA inversion in bacteria

Alternating transcription of two flagillin genes in a Salmonella bacterium is caused by a simple site-specific recombination event that inverts a small DNA segment containing a promoter that in one orientation (A) activates transcription of the H2 flagillin gene as well as a repressor protein that blocks the expression of the H1 flagillin gene. When the promoter is inverted, it no longer turns on H2 or the repressor, and the H1 gene, which is thereby released from repression, is expressed instead (B). The recombination mechanism is activated only rarely (about once every 105 cell divisions). Therefore, the production of one or other flagillin tends to be faithfully inherited in each clone of cells.