GENETICS OF TRANSPOSITION

The genes of transposase and resolvase i.e. tnpR and tnpR are identified by recessive mutations. The above enzymes accomplish the two stages of TnA mediated transposition. Like IS- type elements the transposition stage involves the ends of the elements. A unique feature of TnA family is that a specific internal site is required for resolution.

The mutants of tnpA cannot transpose because the enzyme transposase will not be encoded. However, transposase recognizes the ends of elements and binds to 25 bp long sequence located within 38 bp of the inverted terminal repeat. Transposase also makes the staggered 5 bp long sequence breaks in target DNA where transposon is to be inserted. Resolvase functions in two ways, (i) it acts as repressor of gene expressions, and (ii) provide the resolvase function. The frequency of transposition gets increased in tnpR mutants because tnpR represses the transcription of both tnpA and its own gene. Inactivation of tnpR protein allows the increased synthesis of tnpA resulting in the increased transposition frequency. Therefore, the amount of tnpA transposase is a limiting factor in transposition.

The genes, tnpA and tnpR express divergently from an ATP rich enter-cistronic central region. The effects of tnpR are also medicated by its binding in this region. TnpR resolvase gets involved in recombination between direct repeats of Tn3 in a co integrate structure. But in Tn3, resolution reaction occurs only at a specific site.

The res is the site where the recombination carried by tnpR resolvase occurs. The res site is identified by cis-acting deletions. The deletions block transposition resulting in accumulation of co integrates. The sites bound by tnpR resolvase have been determined by foot printing the DNA- protein complex. It binds independently at each of three sites i.e. I, II and III, each 30-40 bp long. Site I is the region genetically defined as the res site. In the absence of site I, resolution reaction does not proceed. However, resolution also involves binding at sites II and III. In the absence of either of II and III sites reaction proceeds poorly. Site I overlaps with the start point for tnpA transcription and site II with the start point for tnpR transcription.