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Category: Microbial Genetics and Molecular Biology; Bacterial Pathogenesis
Mechanisms of Pilus Antigenic Variation in Neisseria gonorrhoeae, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555818340/9781555810825_Chap08-1.gif /docserver/preview/fulltext/10.1128/9781555818340/9781555810825_Chap08-2.gifAbstract:
The alteration of gene expression by DNA rearrangement appears to be an essential part of most biological systems. When an extremely high level of diversification for a set of gene products with a common function is advantageous, these rearrangements are achieved by the reassortment and recombination of repeated gene segments. The generation of the variable domains in avian immunoglobulins, antigenic variation of surface proteins in African trypanosomes and Borrelia hermsii, and pilus antigenic variation in Neisseria gonorrhoeae each results from the transfer of genetic information from nonexpressed donor alleles or pseudogenes to an active expression locus, a process likened to gene conversion occurring in spore-forming fungi. In this chapter, the author concentrates primarily on recent studies that have influenced understanding of variable pilus expression in gonococci. The vast majority of recombination events between double-stranded DNA forms of the filamentous bacteriophage f1 appear to be a consequence of gene conversion, and the mechanism appears to be the formation of asymmetric heteroduplex molecules. It may also seem that reciprocal exchange and gene conversion demand fundamentally distinct recombination reactions, but mechanistically, these two outcomes may simply be alternative resolutions of the same basic event. The identification of equivalent gonococcal genes by interspecies complementation or by virtue of their DNA homologies and the construction of mutants altered in their expression should make it possible to define their role in the chemistry of the exchange reaction.
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Pilin variation occurring in the absence of transformation. Pilin amino acid sequences (derived from DNA sequencing of pilE-containing plasmid clones) are of P+ revertants arising from the nonpiliated strain NN99 - ClaI during propagation in the presence of DNase I ( 26 ). The arrow indicates the location of the ClaI restriction site encompassing a frameshift mutation in pilE used to demonstrate the nonreciprocal character of the recombination events responsible for reversion. See the text and reference 62 for details. Amino acids are noted by single-letter code. Boxed residues indicate conserved pilin domains. Numbers on the left refer to revertant strain designations, while the numbering of the residues is based on those found in mature pilin. Revertants are ordered to emphasize the use of identical (or highly related) partial pilin gene copies as donors in productive recombination events. Revertants 73, 79, and 83 represent the use of one donor allele, 74 and 85 represent the use of another donor allele, and 78, 86, and 89 represent the use of a third donor allele. Solid underlining denotes the corresponding minimal nucleotide stretches introduced by recombination.
Pilin variation occurring in the absence of transformation. Pilin amino acid sequences (derived from DNA sequencing of pilE-containing plasmid clones) are of P+ revertants arising from the nonpiliated strain NN99 - ClaI during propagation in the presence of DNase I ( 26 ). The arrow indicates the location of the ClaI restriction site encompassing a frameshift mutation in pilE used to demonstrate the nonreciprocal character of the recombination events responsible for reversion. See the text and reference 62 for details. Amino acids are noted by single-letter code. Boxed residues indicate conserved pilin domains. Numbers on the left refer to revertant strain designations, while the numbering of the residues is based on those found in mature pilin. Revertants are ordered to emphasize the use of identical (or highly related) partial pilin gene copies as donors in productive recombination events. Revertants 73, 79, and 83 represent the use of one donor allele, 74 and 85 represent the use of another donor allele, and 78, 86, and 89 represent the use of a third donor allele. Solid underlining denotes the corresponding minimal nucleotide stretches introduced by recombination.