The history and function of the widespread colonization island of Actinobacillus actinomycetemcomitans.

摘要

Genomic islands are clusters of genes with a common history that is distinct and divergent from the histories of the organisms in which they reside. Because genomic islands encode complex and potentially advantageous traits that may be acquired by horizontal transfer, they are major contributors to the evolution and diversification of microbial life.; I have taken an interdisciplinary approach to study a putative genomic island, the Widespread Colonization Island (WCI), which encompasses the tight adherence (tad) locus of genes in the human pathogen Actinobacillus actinomycetemcomitans. Several tad locus genes are known to be required for colonization of surfaces and tenacious biofilm formation in A. actinomycetemcomitans, and similar loci are found in a broad range of Archaea and Bacteria. To rigorously test the hypothesis that the WCI is, indeed, a genomic island encoding genes for colonization and biofilm formation, I used tools and ideas from genetics, microbiology, systematics, and evolutionary biology.; I show that three tad locus genes (rcpC, rcpA, tadZ) are required for nonspecific adherence to surfaces. In combination with previous data, these results confirm that the locus, as a unit, is dedicated to biofilm formation, and it most likely encodes a secretion system for assembly of pili that mediate tight adherence. Further evidence indicates that the product of tadZ, a member of the minD/parA family of genes, is found at the cell pole where it may localize the pilus assembly apparatus.; Superfamily phylogenies show that tad locus genes experienced an evolutionary history independent of genes for related transport systems. I use these phylogenies to reconstruct functional evolution and suggest a classification framework for macromolecular transport. The tad locus appears to be a distinct and anciently diverged subtype of type II macromolecular transport.; The distribution of tad-related loci in microorganisms may be due to both vertical and horizontal inheritance. I use comparative phylogenetic analysis, borrowing techniques for reconstructing co-evolution of hosts and parasites, to infer the evolutionary history of the tad locus in its host genomes. My analysis strongly supports a complex history of gene shuffling by recombination, and multiple horizontal transfers, duplications, and losses of the entire region.