Medical Microbiology

172 3 GeneralBacteriology

Site-specificrecombination.Integrationorexcisionofasequenceinorfrom

targetDNA.OnlyasinglesequenceofafewnucleotidesoftheintegratedDNA

needstobehomologouswiththerecombinationsiteonthetargetDNA.The

integrationofbacteriophagegenomesisanexampleofwhatthisprocessfa-

cilitates(p. 1 84f.)Integrationofseveraldeterminantsofantibioticresistance

inoneintegroncanalsoutilizethisprocess(Fig.3. 19 ).Resistanceintegrons

maybeintegratedintransposableDNA.

Transposition.Thetranspositionprocessdoesnotrequirethedonorandtar-

getDNAtobehomologous.DNAsequencescaneitherbetransposedtoadif-

ferentlocusonthesamemoleculeortoadifferentreplicon.Justasinsite-

specificrecombination,transpositionhasalwaysplayedamajorroleinthe

evolutionofmulti-resistanceplasmids(seeFig.3. 23 ,p. 17 6).

Site-SpecificRecombination(Integron)

Integron

Mobile gene

cassette

Resistance

gene

(without

promoter)

59 bp-element

intI attI sulI

intI attI sulI

intI attIaacC1 orfE aadA2 cmlA sulI

+

PintPant

PintPant

Resistance genes

a

b

Int

egration

Integrase

Ex

cision

PintPant

Fig.3. 19 a Integration or

excisionofacirculargene

cassetteintooroutofanin-

tegron.Anintegronisage-

netic structure containing

thedeterminantsofasite-

specific recombinationsys-

tem.Thisstructureiscapable

of capturing ormobilizing

mobile gene cassettes. It

alsoprovidesthepromoter

fortranscriptionofthecas-

sette genes, which them-

selves have no promoter.

Thecassettesoccurinboth

free circularized and inte-

gratedforms.

intI Integrasegene.Codesforsite-specificintegrase(int=integrase).

attI Attachmentsequencefortheintegrase.Siteofrecombination

(att=attachment).

Pant Promoterfortheintegratedgenecassette(P=promoter).

Pint Promoterfortheintegrasegene.

sulI Geneforsulfonamideresistance(sul=sulfonamide).

59bpIntegrase-specificrecombinationsiteincassette.

bModelofintegron 4 (In 4 ),whichcontainsthreeresistancegenesandan

openreadingframe(orfE),thefunctionofwhichisunknown.In 4 istheresultof

successiveintegrationofseveralresistancegenesattheatt1site.

3

Kayser, Medical Microbiology © 2005 Thieme