Microbial ammonia oxidation is a key and rate-limiting step of the nitrogen cycle. Three previously known ammonia oxidation pathways (i.e., nitrification, anaerobic ammonia oxidation [Anammox], and complete ammonia oxidation [Comammox]) are mediated by autotrophic microbes.

Picolinic acid (PA) is a natural toxic pyridine derivative. Microorganisms can degrade and utilize PA for growth. However, the full catabolic pathway of PA and its physiological and genetic foundation remain unknown. In this study, we identified a gene cluster, designated picRCEDFB4B3B2B1A1A2A3, responsible for the degradation of PA from Alcaligenes faecalis JQ135. Our results suggest that PA degradation pathway occurs as follows: PA was initially 6-hydroxylated to 6-hydroxypicolinic acid (6HPA) by PicA (a PA dehydrogenase). 6HPA was then 3-hydroxylated by PicB, a four-component 6HPA monooxygenase, to form 3,6-dihydroxypicolinic acid (3,6DHPA), which was then converted into 2,5-dihydroxypyridine (2,5DHP) by the decarboxylase PicC. 2,5DHP was further degraded to fumaric acid through PicD (2,5DHP 5,6-dioxygenase), PicE (N-formylmaleamic acid deformylase), PicF (maleamic acid amidohydrolase), and PicG (maleic acid isomerase). Homologous pic gene clusters with diverse organizations were found to be widely distributed in Alpha-, Beta-, and Gammaproteobacteria. Our findings provide new insights into the microbial catabolism of environmental toxic pyridine derivatives. IMPORTANCE Picolinic acid is a common metabolite of L-tryptophan and some aromatic compounds and is an important intermediate in organic chemical synthesis. Although the microbial degradation/detoxification of picolinic acid has been studied for over 50 years, the underlying molecular mechanisms are still unknown. Here, we show that the pic gene cluster is responsible for the complete degradation of picolinic acid. The pic gene cluster was found to be widespread in other Alpha-, Beta-, and Gammaproteobacteria. These findings provide a new perspective for understanding the catabolic mechanisms of picolinic acid in bacteria.

20Picolinic acid (PA) is a natural toxic pyridine derivative. Microorganisms can 21 degrade and utilize PA for growth. However, the full metabolic pathway and its 22 physiological and genetic foundation remain unknown. In this study, we identified the 23 pic gene cluster responsible for the complete degradation of PA from Alcaligenes 24 faecalis JQ135. PA was initially 6-hydroxylated into 6-hydroxypicolinic acid (6HPA) 25 by PA dehydrogenase (PicA). 6HPA was then 3-hydroxylated by a four-component 26 6HPA monooxygenase (PicB) to form 3,6-dihydroxypicolinic acid (3,6DHPA), which 27 was then converted into 2,5-dihydroxypyridine (2,5DHP) by a decarboxylase (PicC). 28 The 2,5DHP was further degraded into fumaric acid, through PicD (2,5DHP 29 dioxygenase), PicE (N-formylmaleamic acid deformylase), PicF (maleamic acid 30 amidohydrolase), and PicG (maleic acid isomerase). Homologous pic gene clusters 31 with diverse organizations were found to be widely distributed in α-, β-, and 32 γ-Proteobacteria. Our findings provide new insights into the microbial metabolism of 33 environmental toxic pyridine derivatives. 34 35 36 3 / 20 Importance 37 Picolinic acid is a common metabolite of L-tryptophan and some aromatic 38 compounds and is an important intermediate of industrial concern. Although the 39 microbial degradation/detoxification of picolinic acid has been studied for over 50 40 years, the underlying molecular mechanisms are still unknown. Here, we show the pic 41 gene cluster responsible for the complete degradation of picolinic acid into the 42 tricarboxylic acid cycle. This gene cluster was found to be widespread in other α-, β-, 43 and γ-Proteobacteria. These findings provide new perspective for understanding the 44 mechanisms of picolinic acid biodegradation in bacteria. 45 46 4 / 20 157 PicF showed high similarities (40~60%) to 2,5DHP 5,6-dioxygenase, NFM 158

The pic gene cluster was found to be responsible for PA degradation and widely distributed in Alpha- , Beta- , and Gammaproteobacteria . Thus, it is very necessary to understand the regulation mechanism of the pic cluster in these strains.

Picolinic acid (PA), a C2-carboxylated pyridine derivative, is a significant intermediate used in industrial production. PA is considered hazardous for the environment and human health. In this study, a Gram-positive bacterium, Rhodococcus sp. PA18, which aerobically utilizes PA as a source of carbon and energy, was isolated. The strain completely degraded 100 mg/L PA within 24 h after induction and formed 6-hydroxypicolinic acid (6HPA), a major PA metabolite, which was identified using ultraviolet-visible spectroscopy, high performance liquid chromatography, and liquid chromatography/time of flight-mass spectrometry analyses. The cell-free extracts converted the PA into 6HPA when phenazine methosulfate was used as an electron acceptor. To our knowledge, this is the first report showing that PA can be metabolized by Rhodococcus. In conclusion, Rhodococcus sp. PA18 may be potentially used for the bioremediation of environments polluted with PA.