EVOLUTION OF NOVEL METABOLIC PATHWAYS FOR THE DEGRADATION OF CHLOROAROMATIC COMPOUNDS

Chlorobenzenes are substrates not easily metabolized by existing bacte ria in the environment. Specific strains, however, have been isolated from polluted environments or in laboratory selection procedures that use chlorobenzenes as their sole carbon and energy source. Genetic ana lysis indicated that these bacteria have acquired a novel combination of previously existing genes. One of these gene clusters contains the genes for an aromatic ring dioxygenase and a dihydrodiol dehydrogenase . The other contains the genes for a chlorocatechol oxidative pathway. Comparison of such gene clusters with those from other aromatics degr ading bacteria reveals that this process of recombining or assembly of existing genetic material must have occurred in many of them. Similar ities of gene functions between pathways suggest that incorporation of existing genetic material has been the most important mechanism of ex panding a metabolic pathway. Only in a few cases a horizontal expansio n, that is acquisition of gene functions to accomodate a wider range o f substrates which are then all transformed in one central pathway, is observed on the genetic level. Evidence is presented indicating that the assembly process may trigger a faster divergence of nearby gene se quences. Further 'fine-tuning',for example by developing a proper regu lation, is then the next step in the adaptation.