TABLE 2.

Recombination efficiency in the presence of RexAB nuclease mutants

pRexAB derivatives in E. coli ΔrecBCD (pΔBla)	Transformation efficiency with pACYC184 (1 μg)	No. of ampicillin-resistant transformants with:a		ChiLl stimulationb
		ChiLl0c	ChiLlc
pRexAB	2 × 106	10	199	30
pRexABD910A	2 × 106	482	3,012	10
pRexABΔDYK	1 × 106	130	309	4
pRexABΔ771–1063	8 × 105	63	112	3
pRexAΔDYKB	5 × 106	8	6	1
pGB2d	2 × 106	30	19	1
TG1 control	1.5 × 107	416	272	1

^aValues represent the total number of transformants obtained in three transformation experiments. For each experiment 200 ng of the appropriate linear DNA was used. 

^bChi_L stimulation was determined as the ratio of ampicillin-resistant transformants obtained with linear DNA fragments containing Chi_Ll compared to those with no Chi_Ll (designated Chi_Ll⁰). To determine the capacity of each fragment to effect homologous recombination, we used conditions of TG1 electroporation that abolish nuclease activity; the ratio obtained in TG1 was used to correct for Chi activity ratios in the test strains (20) (see Materials and Methods). 

^cChi_Ll and Chi_Ll⁰ correspond to the linear fragments used for gene conversion, which do and do not, respectively, contain the double Chi_Ll sites flanking the region of bla homology. 

^dpGB2 was the vector used to clone rexAB genes.