Horizontal gene transfer in bacterial and archaeal complete genomes

There is growing evidence that horizontal gene transfer is a potent evoluti onary Force in prokaryotes, although exactly how potent is not known. We ha ve developed a statistical procedure for predicting whether genes of a comp lete genome have been acquired by horizontal gene transfer. It is based on the analysis of G+C contents, codon usage, amino acid usage, and gene posit ion. When we applied this procedure to 17 bacterial complete genomes and se ven archaeal ones, we found that the percentage of horizontally transferred genes varied From 1.5% to 14.5%. Archaea and nonpathogenic bacteria had th e highest percentages and pathogenic bacteria, except for Mycoplasma genita lium, had the lowest. As reported in the literature, we found that informat ional genes were less likely to be transferred than operational genes. Most of the horizontally transferred genes were only present in one or two line ages. Some of these transferred genes include genes that form part of proph ages, pathogenecity islands, transposases, integrases, recombinases, genes present only in one of the two Helicobacter pylori strains, and regions of genes functionally related. All of these Findings support the important rol e of horizontal gene transfer in the molecular evolution of microorganisms and speciation.