Transposons replicative

Gaps filled (left) or entire transposon replicated (right)... 

Conjugative transposons are self-transmissible large DNA elements (up to 150kbp) located in the donor chromosome. After excision a circular intermediate is formed that is unable to replicate autonomously. It is nicked at a origin of transfer site and one strand is then transferred to the recipient cell. After generating a double-stranded circle the transposon integrates into the recipients chromosome. 

The last group of transposable elements is the DNA transposons that resemble bacterial transposons. They tend to have short life spans within a species. Humans have at least seven families of DNA transposons. Their replication is lessened by the presence of inactive copies, so as the inactive copies accumulate, transposition becomes less efficient. 

Replication fills in the gaps, duplicating the sequences flanking the transposon. 

FIGURE 25-43 Two general pathways for transposition direct (simple) and replicative. The DNA is first cleaved on each side of the transposon, at the sites indicated by arrows. (2) The liberated 31-hydroxyl groups at the ends of the transposon act as nucleophiles in a direct attack on phosphodiester bonds in the target DNA. The target phosphodiester bonds are staggered (not directly across from each other) in the two DNA strands. (3) The transposon is now linked to... 

Replicative transposons. In 1979 Shapiro proposed the mechanism illustrated in Fig. 27-31 for replicative transposons. The two inversely repeated segments (green) at the ends of the transposon are aligned with the recipient DNA whose ends are labeled C and D. In fact, the recombining DNA molecules must be supercoiled.1 613 Staggered cuts are made in the recipient DNA at points a and b, which are 5, 9, or 11 bp apart, depending upon the specific recombinase. Nicks are also made in the transposon ends. The 3 ends from the transposon are resealed... 

When a transposon reacts with another part of the same DNA circle there are two possibilities. The piece of DNA lying between the transposon and the recipient site may be excised as a circle containing a copy of the transposon. Alternatively, there will be inversion of that sequence as well as replication of the transposon (compare with Eqs. 27-15, 27-16). 

Transposition of DNA, which is discussed in Chapter 27, Section D,4, may seem to be a rare and relatively unimportant event in our body cells. However, transposon DNA accounts for 35% or more of the human genome740 and apparently plays a major role in evolution. Like other transposons, the DNA sequences known as retrotransposons also move about within DNA. However, they use an indirect mechanism that involves synthesis of mRNA and reverse transcription.740 741 The reverse transcribed complementary DNA may be inserted back into the genome at new locations. The necessary chemical reactions parallel those involved in the replication of retroviruses (Fig. 28-23, 28-24). Retrotransposens, truncated retrotransposons, and related sequences constitute as much as 16% of the human genome.741... 

S. aureus transposons are small mobile elements that often encode resistance genes (P-lactamase, resistance to erythromycin and tetracycline). All the transposons encode a transposase gene, and the product of this gene catalyses excision and/or replication of the element, as well as integration. Horizontal transfer of transposons to other S. aureus cells is presumably mediated by another MGE that is transferred, most likely a plasmid transferred by transduction or conjugation. Conjugative transposons have also been described in S. aureus. However, it is not clear if native conjugative transposons are found in S. aureus (Novick 1990). 

Transposable elements are mobile DNA segments that can insert themselves into many sites in cellular DNA (7 ). Transposable elements are not capable of autonomous replication but are propagated only when they are inserted into chromosomal or plasmid DNA. Insertion sequence elements (IS elements) are the smallest pieces of DNA (0.7 to 1.5 kb) known to function as transposable elements. IS elements contain only the genes needed for their insertion into cellular DNA. Transposons (Tn elements) are more complex transposable elements that do contain genes unrelated to the insertion function. Transposons frequently have a central piece of DNA containing one or several genes flanked by nearly identical insertion elements at its termini (Figure 2, 8). 

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