Beschreibung:
<jats:title>Abstract</jats:title>
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<jats:title>Background</jats:title>
<jats:p>The lysine, threonine, and methionine biosynthetic pathways share the three initial enzymatic steps, which are referred to as the Common Pathway (CP). In <jats:italic>Escherichia coli</jats:italic> three different aspartokinases (AKI, AKII, AKIII, the products of <jats:italic>thrA</jats:italic>, <jats:italic>metL</jats:italic> and <jats:italic>lysC</jats:italic>, respectively) can perform the first step of the CP. Moreover, two of them (AKI and AKII) are bifunctional, carrying also homoserine dehydrogenasic activity (<jats:italic>hom</jats:italic> product). The second step of the CP is catalyzed by a single aspartate semialdehyde dehydrogenase (ASDH, the product of <jats:italic>asd</jats:italic>). Thus, in the CP of <jats:italic>E. coli</jats:italic> while a single copy of ASDH performs the same reaction for three different metabolic routes, three different AKs perfom a unique step. Why and how such a situation did emerge and maintain? How is it correlated to the different regulatory mechanisms acting on these genes? The aim of this work was to trace the evolutionary pathway leading to the extant scenario in proteobacteria.</jats:p>
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<jats:title>Results</jats:title>
<jats:p>The analysis of the structure, organization, phylogeny, and distribution of <jats:italic>ask</jats:italic> and <jats:italic>hom</jats:italic> genes revealed that the presence of multiple copies of these genes and their fusion events are restricted to the γ-subdivision of proteobacteria. This allowed us to depict a model to explain the evolution of <jats:italic>ask</jats:italic> and <jats:italic>hom</jats:italic> according to which the fused genes are the outcome of a cascade of gene duplication and fusion events that can be traced in the ancestor of γ-proteobacteria. <jats:italic>Moreover</jats:italic>, the appearance of fused genes paralleled the assembly of operons of different sizes, suggesting a strong correlation between the structure and organization of these genes. A statistic analysis of microarray data retrieved from experiments carried out on <jats:italic>E. coli</jats:italic> and <jats:italic>Pseudomonas aeruginosa</jats:italic> was also performed.</jats:p>
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<jats:title>Conclusion</jats:title>
<jats:p>The integration of data concerning gene structure, organization, phylogeny, distribution, and microarray experiments allowed us to depict a model for the evolution of <jats:italic>ask</jats:italic> and <jats:italic>hom</jats:italic> genes in proteobacteria and to suggest a biological significance for the extant scenario.</jats:p>
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