In addition to these intermediates, this strain was capable of growing on several other chemicals such as naphthalene, phenanthrene, 1-naphthol, 1-naphthoic acid, phthalic acid and 4-hydroxybenzoic acid as source of carbon and energy (Table 3). However, it failed to utilize 1-methylnaphthalene, 2-methylnaphthalene, gentisate, protocatechuate, and

ortho and para cresols. Strain PNK-04, isolated from a coal sample, utilizes chrysene as a sole source of carbon and energy. The solubility of chrysene in the aqueous medium is very low, ranging between 1.6 and 2.2 μg L−1. MK 1775 Therefore, its bioavailability is at a critical level (May et al., 1978). For this reason, the ability of strain PNK-04 to degrade chrysene as a sole source of carbon is of environmental significance. To the best of ABT 199 our knowledge, this is the first report on the catabolic pathway of chrysene in a bacterium and specifically in Pseudoxanthomonas species. Based on the identification of metabolic intermediates

and enzymatic data, a tentative catabolic pathway of a chrysene in strain PNK-04 has been proposed. Elucidation of the metabolic pathways of PAH degradation by bacteria has relied heavily on the identification of metabolic intermediates that are excreted into the culture medium during growth and metabolism (Kanaly & Harayama, 2000). Despite reports of bacterial strains that are able to cometabolize chrysene, such as Pseudomonas fluorescens VUN10,011 (Boonchan et al., 1998), Pseudomonas paucimobilis EPA 505 (Mueller et al., 1990) and Stenotrophomonas maltophilia VUN10,010 (Boonchan et al., 1998) or to use it as a sole carbon source for growth, such as Alcaligenes odorans P20 (Demane’che et al., 2004), Pseudomonas fluorescens ZD1839 molecular weight P2a (Caldini et al., 1995; Cenci & Caldini, 1997) and Rhodococcus sp. UW1 (Walter et al., 1991),

no catabolic pathway has so far been elucidated in these organisms. Most of the reports on bacterial degradation of chrysene are confined to initial oxidation. An Escherichia coli strain overexpressing a dioxygenase gene from Sphingomonas sp. CHY-1 is able to convert chrysene to a single product, identified as a cis-dihydrodiol, which presumably corresponds to the initial product of chrysene oxidation by this bacterium (Lal & Khanna, 1996). Baboshin et al. (2008) reported o-hydroxyphenanthroic acid as the subsequent chrysene metabolite in Sphingomonas sp. VKM B-2434, which is the only metabolite accumulated during the cometabolism of chrysene. The initial oxidation of chrysene in strain PNK-04 appears to occur in a similar manner, leading to the formation of hydroxyphenanthroic acid (metabolite C3). Identification of 1-hydroxy-2-naphthoic acid (metabolite C2) indicates that, after decarboxylation and hydroxylation, the hydroxyphenanthroic acid undergoes ring-cleavage and further degradation to produce 1-hydroxy-2-naphthoic acid. The latter is then converted to 1,2-dihydroxynaphthalene by 1-hydroxy-2-naphthoate hydroxylase.