Phase variation

In biology, phase variation is a method for dealing with rapidly varying environments without requiring random mutation.

It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial population.

Although it has been most commonly studied in the context of immune evasion, it is observed in many other areas as well and is employed by various types of bacteria, including Salmonella species.

For this to occur there are typically one or more cofactors (to name a few: DNA-binding proteins and the presence or absence of DNA binding sites) and a site-specific recombinase.

[3] Fimbrial adhesion by the type I fimbriae in E. coli undergoes site specific inversion to regulate the expression of fimA, the major subunit of the pili, depending on the stage of infection.

The FimE recombinase has the capability to only invert the element and turn expression from on to off while FimB can mediate the inversion in both directions.

[4] If excision is precise and the original sequence of DNA is restored, reversible phase variation can be mediated by transposition.

Ag43, located on the cell surface, is encoded by the Agn43 gene (previously designated as flu) and is important for biofilms and infection.

The ON phase of transcription is dependent upon Dam methylating the GATC sequences in the beginning of the Agn43 gene (which happens to overlap with the OxyR binding site).

The promoter sequence can then move between these conserved regions and allow expression of a different gene[citation needed].

[3] Altered gene expression is a result of SSM and depending where the increase or decrease of the short repeat sequences occurs in relation to the promoter will either regulate at the level of transcription or translation.

[5] Translation of a protein can be regulated by SSM if the short repeat sequences are in the coding region of the gene (top portion of the figure).