Polycyclic aromatic compounds are a group of highly recalcitrant organic pollutants. The initial steps in the degradation of polycyclic aromatic hydrocarbon (PAHs) involve the dihydroxylation of the aromatic ring, a step catalysed by dioxygenase enzymes. The degradation of many xenobiotic and hydrocarbon compounds is known to be mediated by plasmid encoded enzymes. In this review, an insight is given into the role of plasmid in degradation of PAHs, acquisition of degradative ability by these organisms via horizontal transfer and clustering, resulting from tranposon-mediated recombination. There is preponderance of information showing high level of plasmid involvement in the degradation of naphthalene and other 2- and 3-ring PAHs. Information on higher molecular weight PAHs is however scanty. Recent studies suggest possible involvement of plasmid in HMWPAH degradation than was previously thought. Many plasmids involved in PAH-degradation are megaplasmids, of linear configuration, encoding part or the whole genes for the complete pathways. In recent times, validation of propositions on degradative gene acquisition by horizontal gene transfer (HGT) has been obtained from field studies. HGT and transposition seems to be more chronologically linked and less fortuitously directed than previously thought. Improvement on the methods used in isolation of degraders and study of these is important, towards making a significant stride in elucidating plasmid involvement in PAH degradation.

Key words: Plasmids, degradation, polycyclic aromatic hydrocarbon.