Silent pilin genes of Neisseria gonorrhoeae MS11 and the occurrence of related hypervariant sequences among other gonococcal isolates

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Abstract
Pilin variation in Neisseria gonorrhoeae depends on a family of variant genes that undergo homologous, intragenic recombination. This work focuses on the repertoire of silent variant pilin genes in strain MS11, which contribute to the extensive variation of the expressed gene copy. A total of 17 silent copies were identified, which are, to varying degrees, truncated at their 5′ coding region and grouped in seven distinct pil loci. Most silent copies belong to loci pilS1, pilS2 and pilS6, which contain six, two and three silent copies, respectively, tandemly arranged. The pilS5 and pilS7 loci each contain only a single copy. In addition, two silent copies are associated with each of the two pilE loci. By comparison with sequences present in the expressed gene of other variants of the same strain, it is suggested that each silent locus is capable of donating variant sequences into the expression locus and, thus, each silent copy can contribute to the variability of pilin expression. Often, concomitant with changes in the expressed copy, the silent copies of the pilE1 locus undergo recombinations as well. Analyses of unrelated clinical isolates of N. gonorrhoeae reveal homologies of hypervariant pilin sequences with those present in strain MS11, suggesting a limited diversity of such sequences within the gonococcal population and the existence of substantial functional constraints on the variability of pilin and pili. The data further indicate that hypervariant pilin sequences are subject to horizontal exchange and interstrain recombination.