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Research Article

ScienceAsia 18 (1992): 003-025 |doi: 10.2306/scienceasia1513-1874.1992.18.003





ABSTRACT: Environmental release of synthetic compounds in the form of herbicides/pesticides, solvents, refrigerants ete. has created major concerns with regard to their effects on human health because of the persistence of many such compounds. The persistence of these compounds is a reflection of the inability of natural microorganisms to utilize them as a sole source of carbon and energy. Many microorganisms can utilize simple chlorinated compounds such as 3-chlorobenzoate (3Cba) or 2,4-dichlorophenoxyacetAte (2, 4-D) as their sole carbon source but cannot utilize higher chlorinated forms such as 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and others. Under strong selection in a chemostAt with 2, 4, 5- T as the only major source of carbon (directed evolution), it has been possible to isolate a strain of Pseudomonas eepacia AC1100 that can utilize 2,4,5-T as its sole source of carbon and energy Molecular cloning of the genes both for 3Cba/2, 4-D as well as 2,4,5-T degradation has shown that while the chlorocatechol (de) genes for 3Cba and 2,4,-D dissimilation are highly homologous, the 2,4,5-T degradative (tft) genes show no homology with any of the genomic DNA of a large number of pseudomonads and other bacteria. Thus, while natural evolution encompasses recruitment of analogous genes and their subsequent divergence to provide new substrate specificity to their gene products, directed evolution such as what occurred in the evolution of tft genes in P. eepacia AC1100 in the chemostat may involve recruitment of totally non-homologous genes from different species or genera in response to short-term, strong selection. The presence of IS931 type of sequences upstream of the chq or tmo gene cluster may additionally suggest that directed evolution may also necessitate the presence of IS elements with strong outwardly-directed promoter activity so as to ensure the expression of newly recruited non-homologous genes in a different cellular background. It would be important to examine other cases of directed evolution to see how widespread such gene recruitment and gene expression mechanisms are in nature.

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Department of Microbiology & Immunology, University of Illinois College of Medicine, P.O. Box 6998, M/C 790 Chicago, IL 60680, U.S.A., and Laboratory of Biotechnology,
1 Chulabhorn Research Institute,
2 Department of Biotechnology, Mahidol University, Bangkok, Thailand.

Received 3 January, 1992