ChlOR, a GMC family oxidoreductase that evolved independently from the actinomycete, confers resistance to amphenicol antibiotics.

Overuse of the amphenicol antibiotics chloramphenicol (CHL) and thiamphenicol (TAP) poses a great threat to ecosystem safety and human health. The strain, Nocardioides sp. LMS-CY, Nocardioides sp. QY071 and Nocardioides sp. L-11A, classified as a gram-positive actinomycete, harbours a complete CHL metabolic pathway. However, the metabolic genes (clusters) involved in the entire pathway in gram-positive actinomycetes are still limited. Here, chlOR _LMS , chlOR _QY071 and chlOR _L-11A completely from the actinomycete Nocardioides spp. were found to act on the C ₁ -OH of the CHL/TAP side chain, directly converting CHL/TAP to 4-nitrobenzaldehyde (PNBD)/4-methylsulfonyl benzaldehyde (PMBD) and transforming PNBD/PMBD into 4-nitrobenzyl alcohol (PNBM)/4-methylsulfonyl phenyl methanol (PMBM). Furthermore, oxidoreductases can transform PNBM into 4-nitrobenzoate (PNBA). The oxidoreductases ChlOR _LMS , ChlOR _QY071 and ChlOR _L-11A were all classified as cellobiose dehydrogenases from the glucose methanol choline (GMC) family. Based on the Swiss-Prot database, ChlOR _QY071 exhibited a lower identity (27.12%-35.10% similarity) with the reported oxidoreductases. Enzymatic and molecular docking analyses showed that ChlOR _QY071 and ChlOR _L-11A from the two similar genomes were remarkably more effective in metabolizing CHL than ChlOR _LMS . Overall, the detailed resistance mechanism of CHL/TAP by actinomycete strains isolated from soil and livestock manure will provide insights into the occurrence of CHL/TAP resistance genes in the environment, resistance risk and bioremediation of CHL/TAP-contaminated environments.
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