ORGANIZATION OF THE TMB CATABOLIC OPERONS OF PSEUDOMONAS-PUTIDA TMB AND EVOLUTIONARY RELATIONSHIP WITH THE XYL OPERONS OF THE TOL PLASMID PWW0

In Pseudomonas putida (Pp) TMB the genes involved in the catabolism of methyl-substituted aromatic hydrocarbons 1,2,4-trimethylbenzene, m- a nd p-xylene (tmb operon), are functionally and genetically homologous to the xyl genes of the plasmid pWW0, but are chromosomally encoded. W e have analysed by cloning, Southern blotting and sequencing of select ed regions the organisation of the tmb cluster. This analysis shows th at the structural and regulatory genes of the tmb and xyl systems exhi bit a high degree of homology and are similarly organised in operons. However the operons are differently arranged on the Pp TMB chromosome and on the pWW0 plasmid. Comparison of the two systems suggests that t he operon arrangement found in pWW0 may have originated from that foun d in Pp TMB via cointegration mediated by replicative transposition or by intermolecular recombination between two copies of the insertion e lement IS1246.