The gene coding for histidase (histidine ammonia-lyase, HAL, EC 4.3.1.3) was isolated from a lambda-EMBL3 genomic library from Pseudomonas putida nicII and subcloned into the expression vector pT7-7. Transformation of Escherichia coli BL21 (DE3) cells with the recombinant vector led to the expression of catalytically active histidase amounting to 20-30% of the total soluble protein in the crude cell extract. A new rapid and highly efficient isolation procedure is described leading to electrophoretically homogeneous histidase within 1.5 days. Six grams of E. coli BL21 (DE3) cells (wet weight) gives approximately 100 mg of homogeneous histidase with a specific activity of 27 IU/mg. To investigate the possible role of serine as a precursor of dehydroalanine in the active site of histidase, each of the four serines, conserved in all known histidases and phenylalanine ammonia-lyases, was consecutively changed to alanine by site-directed mutagenesis. The resulting mutant genes were subcloned into the expression vector pT7-7 and were assayed for histidase activity. The catalytic activities of the four mutants and of wild-type histidase were compared. The Km and Vmax values of the overexpressed mutants S112A, S393A, and S418A and wild-type histidase did not show any significant differences. Mutant S143A, however, was devoid of catalytic activity (< 0.01%), pointing to the outstanding importance of this serine for the formation of an active enzyme. We conclude that serine-143 is the most probable precursor of the active-site dehydroalanine. The role of serine-143 in the biosynthesis of active histidase is discussed.(ABSTRACT TRUNCATED AT 250 WORDS)