Bacterial Degradation of N?-Methylhistidine

The first three enzymatic steps by which organisms degrade histidine are universally conserved. A histidine ammonia-lyase (EC 4.3.1.3) catalyzes 1,2-elimination of the a-amino group from L-histidine; a urocanate hydratase (EC 4.2.1.49) converts urocanate to 4-imidazolone-5-propionate, and this intermediate is hydrolyzed to N-formimino-Lglutamate by an imidazolonepropionase (EC 3.5.2.7). Surprisingly, despite broad distribution in many species from all kingdoms of life, this pathway has rarely served as a template for the evolution of other metabolic processes. The only other known pathway with a similar logic is that of ergothioneine degradation. In this report, we describe a new addition to this exclusive collection. We show that the firmicute Bacillus terra and other soil-dwelling bacteria contain enzymes for the degradation of Nr-methylhistidine to L-glutamate and N-methylformamide. Our results indicate that in some environments, Nr-methylhistidine can accumulate to concentrations that make its efficient degradation a competitive skill. In addition, this process describes the first biogenic source of N-methylformamide.