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Both of the independently isolated TOL plasmids pWW53 and pDK1 contain multiple regions homologous to the xylS regulatory gene of the archetypal TOL plasmid pWW0. The three homologues on pWW53 vary in the extent of their homology to xylSpWW0, xylS1pWW53 is 99% identical to xylSpWW0 and is located relative to the single copy of xylRpWW53 in exactly the same way as xylS and xylR on pWW0. The DNA sequence of xylS3pWW53 is 87% identical to the xylSpWW0 sequence within the coding region but the non-coding DNA upstream is not homologous. There is a frame-shift change at the end of the coding region which causes the C terminus of XylS3pWW53 to be extended by an additional 10 amino acids relative to XylSpWW0. xylS2pWW53 is anomalous and appears to encode a truncated pseudogene lacking the first 525 bases found in the other xylS genes. Evidence is presented to show that both xylS1pWW53 and xylS3pWW53 act as regulators of meta pathway operons. Plasmid pDK1 carries two homologues of xylS. xylS1pDK1 is functional and is a hybrid gene: its 5' end and the upstream sequences are highly homologous to both xylS1pWW53 and xylSpWW0, whereas its 3' end is identical to xylS3pWW53. The sequence of xylS2pDK1 is identical to that of the anomalous truncated xylS2pWW53. Comparison of the organization and the restriction maps of the xyl catabolic operons on pDK1 and pWW53, together with the nucleotide sequences presented here, indicates that the catabolic DNA on pDK1 has derived from a replicon on which the xyl genes are organized similarly to pWW53 and that a genetic rearrangement has taken place involving a reciprocal recombination internal to two of its xylS homologues.

A Comparison of the Multiple Alleles of xylS Carried by TOL Plasmids pWW53 and pDK1 and its Implications for their Evolutionary Relationship