The first stages of Both replicative and non-replicative transpositio are semilar
IS elements, prokaryotic eukaryotic transposons, and bacteriophage Mu .
Donor cut 1. Synapsis stage - two ends of transposon are brought together 3. . Nicked ends joine crosswise;covalent connection between the transposon the target 2. Transposon nicked at both ends; target nicked at both strands
cuts in trans transfers in trans 22 bp Mu integrates by nonreplicative transposition; during lytic cycle- number of copies amplified by replicative transposition - MuA binds to ends as tetramer forming a synapsis . - MuA subunits act in trans to cut next to R1 and L1 (coordinately; two active sites to manipulate DNA). - MuA acts in trans to cut the target site DNA and mediate in trans strand transfer
The chemistry of Donor and target cut The 3’-ends ends groups released from flanking DNA by donor cut reaction They are nuclophile that attack phosphodiester bonds in target DNA
The product of these reaction is strand transfer complex
In strand transfer complex transposon is connected to the target site through one strand at each end
Next step differs and determines the type of transposition:
Strand transfer complex can be target for replication (replicative transposition) or for repair (nonreplicative transposition; breakage & reunion )
transposon target Strand transfer complex
Molecular mechanism of transposition (I) Replicative transposition Replicative transposition proceeds through a cointegrate . Transposition may fuse a donor and recipient replicon into a cointegrate. Resolution releases two replicons-each has copy of the transposon
Replicative transposition Ligation to target ends 3. 3’-ends prime replication The crossover structure contains a single stranded region at each of the staggered ends= pseudoreplication forks that provide template for DNA synthesis Donor and target cut cointegrate .
Non-replicative Replicative additional nicking common structure Breakage & reunion