S trz z

hixL hixR fljB fljA

hixL

inversion hin off rr nrr r

-i hixR

fljB

fljA

invertibie segment neither H2 nor the Hi repressor is synthesized, and the HI-type flagella are present.

Hin Recombination Requires a DNA Enhancer

Hin recombination requires a sequence in addition to the hix sites. This short (^fiO bp] sequence is an enhancer that stimulates the rate of recombination .OOn-fold. Like enhancer sequences that stimulate transcription [see Chapter 17), this sequence can function even when located quite a distance from the recombination sites. Enhancer function requires the bacterial Fis protein [named because it was discovered as a factor for inversion stimulation). Like IHF, Fis is a site-specific DNA bending protein. In addition, it makes protein-protein contacts with Hin thai are important for recombination.

The enhancer-Fis complex activates the catalytic steps of recombination. Hin can actually assemble and pair the hix recombination sites to form a synaptic complex in the absence of Ihe Fis-enhtmcer complex (Figure 11-13). This contrasts to the role of IHF in X integration, where the accessory protein is essential for assembly of the recom-binase-DNA complex. For Fis activation of Hin, the three DNA sites [hixL, hixR, and enhancer) need to come together. Formation of this three-way complex is greatly facilitated by negative DNA

FIGURE 11-13 Complexes formed during Kin-catalyzed recombination.

Hin protein atone recognizes and pairs the two hoc sites. Wen fis protein is atso present, the three-segment complex can form. This complex is called the invertasome, and ts the most active complex for promoting recombination. (Source; From Craig N. et a!. 2002. Mobile DNA II, p. 246. f 9. © 2002 ASM Press.)

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Hin-fi/x synaptic complex

Hin-fi/x synaptic complex

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BioJogicaJ fioic.s of Site-Specific Recombination

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