Bacterial Degradation of τ-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 α-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 -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 and other soil-dwelling bacteria contain enzymes for the degradation of τ-methylhistidine to l-glutamate and -methylformamide. Our results indicate that in some environments, τ-methylhistidine can accumulate to concentrations that make its efficient degradation a competitive skill. In addition, this process describes the first biogenic source of -methylformamide.