A new enantiomer-selective amidase active on several 2-aryl propionamides was identified and purified from a newly isolated Rhodococcus strain. The characterized amidase is an apparent homodimer, each molecule of which has an Mr of 48,554; it has a specific activity of 16.5 g,mol of S(+)-2-phenylpropionic acid formed per min per mg of enzyme from the racemic amide under our conditions. An oligonucleotide probe was deduced from limited peptide information and was used to clone the corresponding gene, named amdA. As expected, significant homologies were found between the amino acid sequences of the enantiomer-selective amidase of Rhodococcus sp., the corresponding enzyme from Brevibacterium sp. strain R312, and several known amidases, thus confirming the existence of a structural class of amidase enzymes. Genes probably coding for the two subunits of a nitrite hydratase, albeit in an inverse order, were found 39 bp downstream of amdA, suggesting that such a genetic organization might be conserved in different microorganisms. Although we failed to express an active Rhodococcus amidase in Escherichia coli, even in conditions allowing the expression of an active R312 enzyme, the high-level expression of the active recombinant enzyme could be demonstrated in Brevibacterium lactofermentum by using a pSRl-derived shuttle vector.After a systematic search for soil microorganisms able to carry out the enantiomer-selective hydrolysis of racemic nitriles or amides, especially 2-aryl propionamides, we recently reported the purification and cloning of such an amidolytic activity from Brevibacterium sp. strain R312 (15). We found that the gene coding for the enantiomer-selective amidase, amdA, was closely linked to the genes coding for the two subunits, a and ,B, of nitrile hydratase, the enzyme responsible for the hydration of various nitriles to the corresponding amides (10). The structure of this region led to the hypothesis that the two genes could be translated from a polycistronic mRNA, thus supporting the view that there is a nitrile utilization operon, at least in this bacterium. These results have been confirmed by another group (9). Moreover, it was found that the identified amidase displays a striking homology to other known amidases, such as the acetamidase from Aspergillus nidulans and the indoleacetamide hydrolases from Pseudomonas syringae (savastanoi) and Agrobacterium tumefaciens. Interestingly, the homology was not scattered through the whole amino acid sequence but was mainly focused in a portion of the sequence around amino acids 150 to 220 of the R312 sequence (47 to 65% strict identity), strongly suggesting that this conserved sequence could be part of the active site of this class of enzymes.We now report the identification of a new Rhodococcus strain, obtained during a microbiological screening of soil samples, which is also characterized by efficient stereospecific amidase activity on 2-aryl propionamides. The enzyme responsible for this activity was purified, cloned, and sequenced. It is significantly ...