Molecular Genetics

The Hin inversion system provides an example of the resolvase-invertase family of site-specific recombinations.

Facts

• Salmonella typhimurium undergoes antigenic phase variation, producing 2 antigenically different flagellar proteins. The chromosomes in phases 1 and 2 differ in the relative orientation of a small chromosomal segment.

• The variation requires a cis-acting hix element at the borders of the invertable element and a trans-acting fljA gene, which encodes a site-specific recombinase, called an invertase.
• The hix target site of the Hin invertase is 26 bp, consisting of a 12 bp sequence separated from its complement by a 2 bp spacer. The spacer sequence can be altered without harming inversion as long as the sequences of both targets are changed.
• Hin inversion also requires Hu and factor FIS binding to a boundary sequence of about 26 bp This sequence can be located 50 to 4000 bp away, but its orientation is important.
• The protein forms a covalent protein-DNA intermediate through a serine hydroxyl and a 5' phosphate. Only intramolecular reactions are catalyzed, yielding inversion.

Interpretations

• The Hin protein belongs to a different class of proteins catalyzing inversions than those of the Flp-Int family: the resolvase-invertase family.

Further information

• The group includes other proteins catalyzing inversion, such as Gin, Cin and Pin. In G inversion the orientation of Mu phage G segment determines tail fibre expression and thus host range (E.coli or Citrobacter). A similar variation in tail fiber expression is due to Cin in bacteriophage P. Pin is responsible for inversion of a segment in an integrated e14 phage.
• The group also includes resolvases that catalyze deconcatenation of DNAs at specific target sites, called "res" sites. The transposon cointegrate resolving enzymes of Tn3 and gamma-delta transposons work this way. Their res sites contain direct repeats instead of inverted repeats used in Hin, Gin and Cin systems.
• Three-dimensional structure studies provide insights into the mechanism used by the Hin recombinase to invert DNA sections.

This is page 3212 of Molecular Genetics by Ulrich Melcher, © 1997, 1998