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Tn Transposons Elements - Many plasmid genes determining drug resistance are known to be located in discrete DNA units. These genes can become dissociated from the plasmids and transposed into another DNA molecule, and from there undergo further translocation. Thus a gene can hop from a drug resistance plasmid to the chromosome, to another plasmid, or to a temperate phage genome. Such movable units of DNA have been called transposons, translocatable elements, transposable elements, and drug resistance determinants.
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The term transposon is used here for the complex drug resistance markers. The simpler transposable elements have been called IS elements. Thus transposons are the more complex transposable elements, which often contain IS elements. They behave like IS elements but contain other genes unrelated to insertion function. They are generally longer than 2 kilobases (kb).
Transposable elements are designated by the symbols Tn1, Tn2, Tn3, etc. The gene function may be indicated in parentheses after the name, e.g. Tn6 (Km) for kanamycin resistance.
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All transposons have repititious sequences at their ends. These sequences are in inverted order, except in Tn9. In Tn9 ISI is present in direct order, with smal1 inverted repeats at the ends.
Tn1, Tn2 and Tn3. The transposon TnA is now termed Tn1, Tn2 or Tn3. depending on the plasmid of origin. Tn] is from the plasmid RP4, Tn2 from RSF 1030 and Tn3 from Rldrd19. These three plasmids are associated with ampicillin resistance (Ap). Penicillin and cephalosporin resistance is due to penicillinase (B lacfam08e), of which two general types are recognized, TEM and 0
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Isoelectric focussing distinguishes two types of TEM β-lactamase, TEM-l and TEM-2. Of the 77 naturally occurring plasmids responsible for ampicllin resistance, 70 produce TEM-I and 7 TEM-2. The enzymes TEM I and TEM-2 are associated with Tn3 and Till, respectively, which show only minor differences in polynucleotide sequence. The TEM protein are essentially similar in different bacterial species.
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TnA is the structural gene for both TEM-l and TEM-2 β-lactamase. It can be transposed from replicon to replicon in a recA-independent manner. TnA is a 4,250 base-pair (3.2* 106 dalton) segment of DNA bounded by inverted complementary sequences of 140 base pairs. TI1A insertions are mutagenic, and occur in two orientations, P and M. Insertions in the P orientation are strongly polar on distal gene expression. Insertions in the M orientation are less polar, and may even have a promoter effect. TnA can be inserted into at least 19 distinct sites in plasmid RSF 1010. It has been suggested that TnA may recognize a specific nuleotide sequence in the recipient genome.
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Tn4. Two types of recombinant plasmids are found in cells containing pSC50 and pSC 101 plasmids. These types result from the translocation of segments of pSC5O onto PSCI01. Type I recombinant plasmid consists of the entire pSCJO1, plus a segment, termed Tn4-(TnS), derived from pSC50. Tn4 determines resistance to streptomycin (Sm), 8ulfonamide (Su) and ampicillin (Ap). It is flanked by a 140-nucJeotide long inverted repeat sequence. The two palindromes ate called IRa and IRb. Insertion of Tn4 into pSCI01 results in the formation of pSCI20 and pSCl79-type recombinant plasmids. Located , on 1n4 are the Tn3 (TnA) elements .Type II recombinant plasmid consists of the entire pSCI01, plus the Tn3 (TnA) transposon which determines resistance to ampicillin (Ap).
Tn5 is the transposable kanamycin resistance element. It consists of a 2,300 base pair segment, with 1,450 base pair reverse-oriented repeated sequences at the two ends. It can be transposed from the lambda phage to the E.coli chromosome and from one chromosome to another. The TnS element causes mutation by insertion into genes. The lacZ gene has 30 sites, of TnS insertion. TnS-induced mutations can revert by excision of Tn5 of the 20 1acZ Tn5 insertion mutation studied,19 reverted to lacZ + . 99% of the revertants were Kans, indicating that the Tn5 excision is not associated with the transposition of Tn5 to new sites.Tn6 determines kanamycin resistance, and is about 4,100 base pairs long.
Tn7 (TnC) is an 8.5 X 106 dalton DNA derived fromR483. It encodes resistance to trimethoprim (Tp) and streptomycin (Sm). It is freely transposable to other replicon in a rec A-independent manner.
Tn9 is the transposon which carries an R-factor-derived gene (cam) for chloramphenicol resistance. The structural gene for chloramphenicol acetyl tran8ferase (CAT), the enzyme which confers drug resistance, consists of an 890 base pair segment in the middle or Th9, with insertion sequence IS1on either side. The IS1element consists of about 800 base pairs. The repititious sequences (IS1) of Tn9 are in direct order, and not the inverted repeats found in other-transposons. It is likely that the IS1 sequences are the operative elements in the translocation of Tn9.The chloramphenicol resistance element is translocated as a unit from an R factor (drug resistance plasmid) via the phage PI into the F episome as well as into the lambda phage. Lambda particles carrying Tn9 are referred as to ? cam. Tn9 differs from other transposons in its instability. It suffers a high-frequency loss of its antibiotic resistance ability.
Two different mechanisms appear to operate for this loss. In the first mechanism the repeated sequences bracketing the cam gene are acted upon by the known recombination systems. In the second mechanism Tn9 promotes deletions of itself and 1 or of neighbouring genes.
Tn10 carries the genes for tetracycline resistance, and has inverted repeat sequences of about 1,400 base pairs on either side. These sequences appear to be IS3, elements which cooperate to bring about the translocation of the tetracycline resistance determinant. Tn10 can translocate as a distinct unit into several different kinds of prokaryotic genomes. Botstein and Kleckner (1977) have followed the translocation of To10 from R222 {a drug-resistance plasmid) to the genome of P22 (a Salmonella phage), where there are at least 20 different sites of insertion, from P22 into the Salmonella chromosome (at least a 100 different insertion sites) from Salmonella to the genome of coliphage lambda (at least 5 sites); and finally from lambda to the E. coli chromosome (at least 20 sites)
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