Summary: Both of the independently isolated TOL plasmids pWW53 and pDK1 contain multiple regions homologous to the regulatory gene of the archetypal TOL plasmid pWW0. The three homologues on pWW53 vary in the extent of their homology to . is 99% identical to and is located relative to the single copy of in exactly the same way as and on pWW0. The DNA sequence of is 87% identical to the 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 XylS3 to be extended by an additional 10 amino acids relative to XylS. is anomalous and appears to encode a truncated pseudogene lacking the first 525 bases found in the other genes. Evidence is presented to show that both and act as regulators of pathway operons. Plasmid pDK1 carries two homologues of . is functional and is a hybrid gene: its 5” end and the upstream sequences are highly homologous to both and , whereas its 3” end is identical to . The sequence of is identical to that of the anomalous truncated . Comparison of the organization and the restriction maps of the 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 genes are organized similarly to pWW53 and that a genetic rearrangement has taken place involving a reciprocal recombination internal to two of its homologues.


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