3-Hydroxypyridine Dehydrogenase HpdA Is Encoded by a Novel Four-Component Gene Cluster and Catalyzes the First Step of 3-Hydroxypyridine Catabolism in Ensifer adhaerens HP1.

3-Hydroxypyridine (3HP) is an important natural pyridine derivative. Ensifer adhaerens HP1 can utilize 3HP as its sole sources of carbon, nitrogen, and energy to grow, but the genes responsible for the degradation of 3HP remain unknown. In this study, we predicted that a gene cluster, designated 3hpd, might be responsible for the degradation of 3HP. The analysis showed that the initial hydroxylation of 3HP in E. adhaerens HP1 was catalyzed by a four-component dehydrogenase (HpdA1A2A3A4) and led to the formation of 2,5-dihydroxypyridine (2,5-DHP). In addition, the SRPBCC component in HpdA existed as a separate subunit, which is different from other SRPBCC-containing molybdohydroxylases acting on N-heterocyclic aromatic compounds. Moreover, the results demonstrated that the phosphoenolpyruvate (PEP)-utilizing protein and pyruvate-phosphate dikinase were involved in the HpdA activity, and the presence of the gene cluster 3hpd was discovered in the genomes of diverse microbial strains. Our findings provide a better understanding of the microbial degradation of pyridine derivatives in nature and indicated that further research on the origin of the discovered four-component dehydrogenase with a separate SRPBCC domain and the function of PEP-utilizing protein and pyruvate-phosphate dikinase might be of great significance.IMPORTANCE 3-Hydroxypyridine is an important building block for the synthesis of drugs, herbicides, and antibiotics. Although the microbial degradation of 3-hydroxypyridine has been studied for many years, the molecular mechanisms remain unclear. Here, we show that 3hpd is responsible for the catabolism of 3-hydroxypyridine. The 3hpd gene cluster was found to be widespread in Actinobacteria, Rubrobacteria, Thermoleophilia, and Alpha-, Beta-, and Gammaproteobacteria, and the genetic organization of the 3hpd gene clusters in these bacteria shows high diversity. Our findings provide new insight into the catabolism of 3-hydroxypyridine in bacteria.