DNA-binding proteins in cells and membrane blebs of Neisseria gonorrhoeae

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Abstract
Naturally elaborated membrane bleb fractions BI and BII of Neisseria gonorrhoeae contain both linear and circular DNAs. Because little is known about the interactions between DNA and blebs, studies were initiated to identify specific proteins that bind DNA in elaborated membrane blebs. Western immunoblots of whole-cell and bleb proteins from transformation-competent and DNA-uptake-deficient (dud) mutants were probed with single- or double-stranded gonococcal DNA, pBR322, or synthetic DNA oligomers containing intact or altered gonococcal transformation uptake sequences. The specificity and sensitivity of a nonradioactive DNA-binding protein assay was evaluated, and the assay was used to visualize DNA-protein complexes on the blots. The complexes were then characterized by molecular mass, DNA-binding specificity, and expression in bleb fractions. The assay effectively detected blotted DNA-binding proteins. At least 17 gonococcal DNA-binding proteins were identified; unique subsets occurred in BI and BII. Certain DNA-binding proteins had varied affinities for single- and double-stranded DNA, and the intact transformation uptake sequence competitively displaced the altered sequence from a BI protein at 11 kilodaltons (kDa). A dud mutant, strain FA660, lacked DNA-binding activity at the 11-kDa protein in BI. The segregation of DNA-binding proteins within BI and BII correlates with their distinct protein profiles and suggests that these vesicles may play different roles. Although the DNA-binding proteins expressed in BII may influence the nuclease-resistant export of plasmids within BII vesicles, the BI 11-kDa protein may bind transforming DNA.