Medical Microbiology

TheGeneticVariabilityofBacteria 173

DetailsofTransposableDNAElements

&Insertionsequences(ISelements,Fig.3. 20 ).Thesearethesimplesttrans-

posableDNAsequences.Theyareterminatedbyidentical,butreversed,sequences

of 10 – 40 nucleotidesknownasinvertedrepeats(IR).Theyframethesegmentthat

codesfortheenzymetransposase.Thetargetstructuresforthisenzymearetheso-

calleddirectrepeats,nucleotidesequencescomprising 5 – 9 bpthatareduplicatedin

theintegrationprocess.

&Tn3transposons(Fig.3.20b).InadditiontothetransposasegenetnpA,they

containtheregulatorsequencetnpRandtheressitetowhichresolvasemust

bind.Tn3-liketransposonsareduplicatedinthetranspositionprocess,sothat

onecopyremainsattheoriginallocationandtheotherisintegratedatthenew

location.

&Compositetransposons(Fig.3.20c).TheyconsistoftwoISelementsframinga

sequenceofvariablesizethatisnotrequiredfortransposition,e.g.,aresistance

gene.

&Conjugativetransposons(Fig.3.20d).Thesegeneticelementscodeincertain

regionsforfactorsthatcontrolthetransfer(Tra)andtransposition(Tn)processes.

ConjugativetransposonshavebeendiscoveredmainlyinGram-positivecocciand

Gram-negativeanaerobes(Bacteroides).

TransposableDNAElements

tnp

a

IS elements

800– 1 500 bp

tnp

b

Tn3-like transposons

Transposon 3 (Tn 3 ), 4.9 kbp,

codes for betalactamase (= bla)

tnpR res bla

IS10L

c

Composite transposons

Transposon 10 (Tn 10 ), 9.3 kbp, tetB = determinant

of resistance to tetracyclines

tetB IS10R

tn

d

Conjugative transposons

Transposon 916 (Tn 916 ), 18 kbp, determinant of resistance to tetracyclines (tetM)

tetM

Trans-

posase-

gene

Inverted

repeats

Direct repeats

tra tn tra tn tra tntra tra

Fig.3. 20 a–dExplanationofthestructuresandabbreviationsusedinthetext

box“DetailsofTransposableDNAelements”below.

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Kayser, Medical Microbiology © 2005 Thieme