BETA-RECOMBINASE CATALYZES INVERSION AND RESOLUTION BETWEEN 2 INVERSELY ORIENTED 6 SITES ON A SUPERCOILED DNA SUBSTRATE AND ONLY INVERSION ON RELAXED OR LINEAR SUBSTRATES

The beta recombinase, in the presence of a chromatin-associated protei n such as Hbsu, catalyzes DNA resolution or DNA inversion on supercoil ed substrates containing two directly or inversely oriented six sites. Hbsu stabilizes the formation of the recombination complex (Alonso, J , C,, Weise, F,, and Rojo, F, (1995) J, Biol, Chem. 270, 2938-2945), I n this study we show that resolution by beta recombinase strictly requ ires supercoiled DNA, but inversion does not, On a substrate with two inversely oriented six sites, beta recombinase catalyzed both resoluti on and inversion if the DNA was supercoiled but only inversion if the substrate was relaxed or linear. Hbsu was critical for the formation o f synaptic complexes; its concentration relative to that of the superc oiled DNA substrate determined whether resolution or inversion product s were preferentially formed. The results suggest that the beta recomb inase forms unproductive short-lived synaptic complexes between two ju xtaposed inversely oriented six sites; the presence of 3 to 13 Hbsu di mers per supercoiled DNA molecule would stabilize a synaptic complex w ith a relative geometry of the six sites allowing beta recombinase pre ferentially to achieve resolution. Supercoiling probably helps to over come an energetic barrier, since resolution does not occur in relaxed DNA. The presence of >30 Hbsu dimers per DNA molecule probably favors the formation of a recombination complex with a different geometry sin ce the reaction is directed preferentially toward DNA inversion.