Two identical direct repeats of a 1275 bp sequence, designated IS1246, encompass the xyl genes, which determine the catabolism of toluene, m- and p-xylenes to central metabolites, on the TOL catabolic plasmid pWW0. IS1246 has a terminal inverted repeat of 12 bp (5'GGGCACCTCGAA3') and contains a major open reading frame of 280 codons. This ORF shows significant homology with ORFs encoded by a number of bacterial insertion sequences from Bacteroides, Neisseria and Escherichia coli.