The fermentation of β‐alanine by Clostridium propionicum proceeds via activation to the CoA‐thiol ester, followed by deamination to acryloyl‐CoA, which is also an intermediate in the fermentation of l‐alanine. By shifting the organism from the carbon and energy source α‐alanine to β‐alanine, the enzyme β‐alanyl‐CoA:ammonia lyase is induced 300‐fold (≈ 30% of the soluble protein). The low basal lyase activity is encoded by the acl1 gene, whereas the almost identical acl2 gene (six amino acid substitutions) is responsible for the high activity after growth on β‐alanine. The deduced β‐alanyl‐CoA:ammonia lyase proteins are related to putative β‐aminobutyryl‐CoA ammonia lyases involved in lysine fermentation and found in the genomes of several anaerobic bacteria. β‐Alanyl‐CoA:ammonia lyase 2 was purified to homogeneity and characterized as a heteropentamer composed of 16 kDa subunits. The apparent Km value for acryloyl‐CoA was measured as 23 ± 4 µm, independent of the concentration of the second substrate ammonia; kcat/Km was calculated as 107 m−1·s−1. The apparent Km for ammonia was much higher, 70 ± 5 mm at 150 µm acryloyl‐CoA with a much lower kcat/Km of 4 × 103 m−1·s−1. In the reverse reaction, a Km of 210 ± 30 µM was obtained for β‐alanyl‐CoA. The elimination of ammonia was inhibited by 70% at 100 mm ammonium chloride. The content of β‐alanyl‐CoA:ammonia lyase in β‐alanine grown cells is about 100 times higher than that required to sustain the growth rate of the organism. It is therefore suggested that the enzyme is needed to bind acryloyl‐CoA, in order to keep the toxic free form at a very low level. A formula was derived for the calculation of isomerization equilibra between l‐alanine/β‐alanine or d‐lactate/3‐hydroxypropionate.

Wild-type Salmonella typhimurium expresses three proline transport systems : a high-affinity proline transport system encoded by the putP gene, and two glycine betaine transport systems with a lower affinity for proline encoded by the prop and proU genes. Although proline uptake by the Prop and ProU transport systems is sufficient to supplement a proline auxotroph, it is not efficient enough to allow proline utilization as a sole source of carbon or nitrogen. Thus, the PutP transport system is required for utilization of proline as a carbon or nitrogen source. In this study, an overexpression suppressor, designated pmY, which allows proline utilization in a putP genetic background and does not require the function of any of the known proline transport systems, was cloned and characterized. The suppressor gene, designated proY, maps at 8 min on the 5. typhimurium linkage map, distant from any of the other characterized proline transport genes. The DNA sequence of the proY gene predicts that it encodes a hydrophobic integral membrane protein, with strong similarity to a family of amino acid transporters. The suppressor phenotype requires either a multicopy clone of the pmY+ gene or both a single copy of the p r o p gene and a pmZ mutation. These results indicate that the proY gene is the structural gene for a cryptic proline transporter that is silent unless overexpressed on a multicopy plasmid or activated by a proZ mutation.

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