TABLE 3.

Mobilization frequencies of the mutated transfer origins^a

Plasmid testedb	Mob-producing plasmid in trans	Mobilization frequency of the plasmid testedc
poriTA		<10−8
poriTC		<10−8
pBBR1CM		1 × 10−1
pBBR1MCS2		1 × 10−1
poriTA	pBBR1CM	<10−8
poriTC	pBBR1CM	<10−8
pJL52bp	poriTA	9 × 10−5
pJL52bp	poriTC	1 × 10−3
pJL52bp	pBBR1MCS2	8 × 10−2

^aFor the mobilization experiments, Nal^s S17-1 bacteria containing the plasmids to be tested were used as donors and the Nal^r F⁻ strain XA106 (from our laboratory collection) was used as a recipient. After overnight incubation at 30°C, the transconjugants were selected at 37°C on LB plates supplemented with nalidixic acid (20 μg/ml) and the antibiotic allowing selection of bacteria with the mobilizable plasmid (kanamycin at 50 μg/ml or chloramphenicol at 15 μg/ml). 

^bThe poriTA and poriTC plasmids contain the mutated oriT and the wild-type mob gene. Because the mutations were in the promoter sequence, the pBBR1CM plasmid was used to produce the Mob protein in trans, and the pJL52bp plasmid containing the wild-type oriT was used to test expression of the mob gene from poriTA or poriTC. The pBBR1MCS2 plasmid is a kanamycin-resistant derivative of pBBR1 and was used as a positive control. 

^cCalculated as the ratio of the number of transconjugants to the number of donors. Each value is an average from three independent experiments. The limit of detection of transconjugants was estimated at 10⁻⁸.