..... In the following diagram of a "Tn element", specifically Tn10, we see that it is already inserted in the host's DNA, that is shown at each end. .. Now let's take a tour of the whole thing starting with the lefthand host DNA and going "eastward" to eventually arrive at the other part of the host-DNA. .. This host, by the way, is the generalized E.coli (which includes Salmonella and Shigella because many bacteriologists, such as the Copenhagen WHO typists, consider E.coli, Salmonella and Shigella really just one species as their genetic maps are 99.9% identical).

GUIDED TOUR

1. GENERAL OVERVIEW
   1. Somewhere (see later) on the host DNA there is a genetic sequence into which the transposon can integrate. ..This is not a allelic thing, but is rather like the phage lambda's integration. ..The host has a target sequence, and the "mobile element" (Tn10 or lambda) has its own sequence which will be involved in the crossover needed to integrate its own small circular bit of DNA into that of the host DNA. ..In the case of lambda phage, that sequence is called the "prophage ends", and in the case of Tn's, that integration sequence is called the "insertion sequence" or "IS".

   2. As each Tn has its own specific type of IS, both the Tn and the IS have the same number designation. ..Thus Tn10 has IS10. .. On the above diagram, the direction of the "polar" IS10 (meaning it can be transcribed only one way) is antiparallel to the one found at the other end of the integrated Tn. ..Thus it is written backwards at the left end and the other way at the other end.
   
   

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2. A CLOSER LOOK
   1. Interestingly, IS is a gene that codes for a repressor; hence, the "R".

   2. The little circle indicates a promoter, which is the landing site for transcriptase. .. Every expressible gene must have a "p". ..You see that Tn10 has five of them, and that the host's transcriptases will start in the center of the Tn10 and read outwards in both directions - those reading leftwards are reading "Watson", and those reading rightwards are reading "Crick."

   3. Next we see the gene for the enzyme Transposase, which is critical for "jumping". .. This enzyme has the dual function of being both an excisionase and an integrase. ..What more? The future will tell!

   4. We next come to the gene for Resolvase, which is critical for smoothing out the recombination. .. When a circular DNA integrates into a another circle, kinks can happen unless there can be a relaxation of one of the twists in the double helix. ..Sometimes these enzymes are called helicases or gyrases.

   5. Then comes ß-lactamase, which can degrade intracellular tetracycline, and confers tet^R to the host.

   6. Finally comes another IS10, and then the host.

   7. Where Tn10 inserts into the E.coli genome is always were there is the AATTC/TTAAG sequence as shown.

..... When Tn10 decides to leave, it leaves behind a small amount of its IS10 ends as little sticky ends that help the host DNA to come together and "heal" via "ligase" action. ..However, the short piece that is left behind (a "footprint") will continue to interrupt the gene into which the Tn10 had settled. ..As geological time goes on, footprints accumulate, and since the footprints have hints of IS activity, occasionally a Tn will excise improperly such that it is NOT the nearest pair of IS's that will be the end, but another IS further down the line might be involved. Suddenly the Tn gains a gene or two of the host's DNA and jumps with them to another site.

..... AND THAT'S JUST IT! .. Another site may be elsewhere on the host's DNA, but it can also be on a virus that happens to be around. .. Now that virus - such as lambda - will suddenly have some interesting qualities: it will carry the tet^R and one or two host genes to another cell, where the enhanced Tn will jump off the viral genome and onto the new host's genome. ..(This is like transduction in lambda phage. .. Because viruses are usually very restricted in the amount of DNA that they can have packaged in their "heads", for a phage to carry those Tn's, some phage genes don't get packaged, and the virus is thus unable to replicate in the new host. .. Hence, Tn's lethally parasitize the virus in this transduction.)

..... Once the enhanced Tn integrates into the new host's genome, it may later excise using the nearest pair of IS's leaving the former host's somatic genes functional even though in a strange location on the genome.

..... The technological value of all this? .. You can jump normal genes from Host A over to Host B that is a multiple mutant. By leaving out a nutrient, and including tetracycline in the medium, any colonies that grow up must contain both tet^R AND the mobilized Host A gene. ..Then, from the zillions of cells in that colony, you can very likely find another excision that moves tet^R along with two somatic genes - one from Host A and one or more from Host B. ..The frequency of this mobilization of Host B's genes can be used to tell the closeness of the two Host B genes. .. Now, when I talk about Host B genes, I might be talking about two mere mutations within a gene. ..And here the frequencies can be determined down to about 5 base pairs. ..Welcome to the world of fine structure mapping!

..... When this author was in Edinburgh, "jumping" the lac-operon to various places on the E.coli map, several people in other labs ran off with my strains (that on the surface appeared to have the lac-operon inserted in the same place) and they used my new strains to map the 'termination' region in E.coli ("ter" is opposite to "oriC", which is the origin of Chromosome replication in E.coli). ..So my project there was designed to accomplish my work as well as to feed another group's work.

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