Histidine ammonia-lyase (EC 184.108.40.206) catalyzes the nonoxidative elimination of the alpha-amino group of histidine and is closely related to the important plant enzyme phenylalanine ammonia-lyase. The crystal structure of histidase from Pseudomonas putida was determined at 2.1 A resolution revealing a homotetramer with D2 symmetry, the molecular center of which is formed by 20 nearly parallel alpha-helices. The chain fold, but not the sequence, resembles those of fumarase C and related proteins. The structure shows that the reactive electrophile is a 4-methylidene-imidazole-5-one, which is formed autocatalytically by cyclization and dehydration of residues 142-144 with the sequence Ala-Ser-Gly. With respect to the first dehydration step, this modification resembles the chromophore of the green fluorescent protein. The active center is clearly established by the modification and by mutations. The observed geometry allowed us to model the bound substrate at a high confidence level. A reaction mechanism is proposed.