Molecular Genetics

Some transposons form and resolve cointegrates.

Facts

• The transposon Tn3, one of the best characterized TnA family members, has three protein encoding genes, identified genetically and by sequence analysis. It has 38 bp inverted repeat termini.

• The bla gene is a gene for resistance to ampicillin-type antibiotics.
• Mutations in tnpA are recessive and prevent transposition. The tnpA protein binds to 25 bp within the inverted repeat termini.

Some mutations in tnpR are also recessive and result in enhanced transposition. The tnpR protein binds to three similar sites between tnpA and tnpR coding regions. It is a site-specific recombinase of the resolvase-integrase family. Mutations concentrated at one tnpR binding site block completion of transposition. These res mutations result in the accumulation of cointegrates. A cointegrate is a fusion of donor and recipient replicons with one element at each border between replicons.

Interpretations

• The tnpA protein is a transposase that initiates transposition by making single-stranded nicks at the ends of the transposon and in a target sequence. Replication processes proceed from the 3' OH ends to produce two copies of the transposon and the fusion of donor and recipient sequences. This transposition process is called replicative transposition.
• Donor and recipient sequences in the cointegrate are separated by a site-specfic recombination reaction catalyzed by the tnpR protein. The result is separated donor and recipient replicons each with a transposon, and is called cointegrate resolution.
• The tnpR protein, by binding near the promoters of both tnpA and tnpR, also acts as a repressor of transcription, thus limiting transposition.

Further information

• Tn1000 is another well studied member of the TnA family.
• Cointegrating forming transposons differ from composite transposons.
• Transposon movement is regulated to prevent uncontrolled transposition.

This is page 32213 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2000