Rhodococcus rhodochrous PY11 (DSM 101666) is able to use 2-hydroxypyridine as a sole source of carbon and energy. By investigating a gene cluster (hpo) from this bacterium, we were able to reconstruct the catabolic pathway of 2-hydroxypyridine degradation. Here, we report that in Rhodococcus rhodochrous PY11, the initial hydroxylation of 2-hydroxypyridine is catalyzed by a four-component dioxygenase (HpoBCDF). A product of the dioxygenase reaction (3,6-dihydroxy-1,2,3,6-tetrahydropyridin-2-one) is further oxidized by HpoE to 2,3,6-trihydroxypyridine, which spontaneously forms a blue pigment. In addition, we show that the subsequent 2,3,6-trihydroxypyridine ring opening is catalyzed by the hypothetical cyclase HpoH. The final products of 2-hydroxypyridine degradation in Rhodococcus rhodochrous PY11 are ammonium ion and ␣-ketoglutarate.
Pyridine and its derivatives are ubiquitous in nature. The pyridine ring is found in alkaloids (e.g., nicotine, actinidine), coenzymes [NAD(P)H, pyridoxal], and man-made solvents, pesticides, and herbicides (e.g., paraquat). Hydroxypyridines are common intermediate metabolites produced during microbial biodegradation of various N-heterocycles (pyridine, nicotine, picoline, 2,6-dipicolinic acid) (1-3).It has previously been reported that Arthrobacter crystallopoietes, Arthrobacter pyridinolis, and Arthrobacter viridescens (4), Achromobacter sp. strain G2 (5), and Nocardia sp. strain PNO (6) use 2-hydroxypyridine (2HP) as a sole carbon and energy source. Through more than 50 years of investigation of pyridine ring metabolism, many intermediates have been identified and metabolic pathways have been proposed. However, the genes and enzymes responsible for 2HP biodegradation have seldom been reported.In Achromobacter sp. G2, 2HP is metabolized via the maleamate pathway (5) (Fig. 1). No enzymes responsible for the initial hydroxylation step of 2HP leading to the formation of 2,5-dihydroxypyridine (2,5DHP) have been reported to date. Nevertheless, the degradation of 2,5DHP, an intermediate of nicotinic acid metabolism, has been fully investigated by Jiménez et al. (7), and all genes encoding the enzymes involved in the maleamate pathway have been identified and characterized (7).Arthrobacter crystallopoietes, A. pyridinolis, and A. viridescens (4) and Arthrobacter sp. strain PY22 (8) produce a blue pigment (nicotine blue) in the medium when grown on 2HP. The nicotine blue has been shown to be a 4,5,4=,5=-tetrahydroxy-3,3=-diazadiphenoquinone-(2,2=) (9) that is an autoxidation product of 2,3,6-trihydroxypyridine (THP). THP can be synthesized via hydroxylation of 2,5DHP, 2,3-dihydroxypyridine (2,3DHP), or 2,6-dihydroxypyridine (2,6DHP); however, only the 2,6DHP 3-hydroxylase, which is involved in the biodegradation of nicotine by Arthrobacter nicotinovorans, has been identified to date (10).We have previously reported that HpyB monooxygenase from Arthrobacter sp. PY22 is sufficient for the conversion of 2HP to THP (8). Since no reaction intermediates have been detected, a consecutive two-st...