The genes for benzene catabolism in Pseudomonas putida ML2 are flanked by two copies of the insertion element IS1489, forming a class-I-type catabolic transposon, Tn5542

Abstract

Two directly repeated sequences of the IS elements IS1489v1 and IS1489v2 flank the benzene dioxygenase (bedC1C2BA) and the cis-benzene dihydrodiol dehydrogenase (bedD) genes on the catabolic plasmid pHMT112 in Pseudomonas putida ML2, forming a Class-I-type composite transposon, Tn5542. Both IS1489v1 and IS1489v2 contain an identical 1371-bp open reading frame, tnpA, that is preceded by a possible ribosome binding site. The tnpA gene of IS1489v1 is bound by a pair of 40-bp imperfect inverted repeats while that of IS1489v2 is flanked only by the left inverted repeat. The tnpA gene codes for a putative 53-kDa polypeptide of 456 amino acids bearing similarity to transposases encoded on IS elements of P. alcaligenes, P. aeruginosa, P. stutzeri, and Serratia marcescens. The basic nature of the putative TnpA protein with a deduced pI of 8.93 is typical of IS-encoded transposases. Similar to other IS elements, an outward facing promoter was detected at the right end of IS1489v1. Experiments involving the suicide vector, pKNG101, failed to show transposition of Tn5542.