A DNA transformation dose-response curve of piliated (P+) gonococci with the use of cloned DNA containing a pilE2-cat fusion showed saturation at high and low levels of transforming DNA. At low DNA concentrations, transformation of the P+ strain MS11-A was effectively inhibited by a 1000-fold molar excess of the gonococcal transformation uptake sequence (GCUS). The same molar excess of the GCUS did not inhibit transformation of MS11-A at high DNA concentrations. In MS11-B2, a nonpiliated (P-), pilin-nonproducing, isogenic variant of MS11-A, the GCUS did not inhibit transformation at any level of transforming DNA. These data suggest that two mechanisms of transformation exist in P+ cells: one which utilizes the GCUS and one which does not. In MS11-B2 P- cells, no evidence was found for the presence of the GCUS-dependent mechanism, suggesting that transformation in this background occurs solely by the GCUS-independent mechanism.
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