MT15 contains a large plasmid, pWW15, of about 250 kbp, which encodes the genes for toluene and xylene catabolism. Growth on benzoate selects strongly against the wild-type and results in the segregation of three phenotypically distinguishable mutant types. (1) BU mutants, which have lost the complete plasmid. (2) B3 mutants, in which the plasmid has undergone a large deletion of about 90 kbp which appears to affect the regulation of the catabolic enzymes; these mutants retain the ability to grow on m-xylene and toluene (MxyTln) but no longer grow on the metabolite of m-xylene, m-toluate (Mtol). (3) A novel class not previously described, the B5 mutants, which still grow well on toluene but grow very poorly on m-xylene and do not grow on m-toluate (Mxy Tln Mtol). The B5 mutants appear to share the regulatory lesion of the B3 mutants but in addition do not express the xylF and xylG gene products, 2-hydroxymuconic semialdehyde hydrolase and 2-hydroxymuconic semialdehyde dehydrogenase. The plasmids in the B5 mutants have also undergone a deletion of about 90 kbp similar to, but distinguishable from, that in the B3 mutants.

Both B3 and B5 mutants can revert to growth on m-toluate. The revertants all show elevated constitutive levels of catechol 2,3-oxygenase, 2-hydroxymuconic semialdehyde dehydrogenase and 2-hydroxymuconic semialdehyde hydrolase which are not further induced by m-toluate. The reversion is accompanied by the tandem amplification of a region of 23-28 kbp on either side of the original deletion. As a result of Southern hybridizations, it was shown that the amplified region contains the structural genes of some of the enzymes which metabolize m-toluate but not the enzymes which convert m-xylene to m-toluate.


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