Directed evolution of amidase in Methylophilus methylotrophus; purification and properties of amidases from wild-type and mutant strains

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Abstract
The obligately methylotrophic bacterium Methylophilusmethylotrophus hydrolyses acetamide and acrylamide using a cytoplasmic amidase. In previous work, continuous culture was used to isolate spontaneous mutants which overexpressed either the wild-type amidase (strain MM6) or a mutant amidase with an apparently higher K cat (strain MM8). We now report that NTG mutagenesis of strain MM8 followed by acrylamide-limited growth at low dilution rate (D 0·025 h-1; 37 °C) led to the selection of a strain which continued to overexpress the amidase, but which exhibited an unexpectedly low amidase activity and a greatly decreased K m for acrylamide (strain MM15). Amidases from the wild-type and mutant strains were purified and shown to be homotetramers (subunit M r 38000, pI 4·1). The N-terminal amino acid sequence of the wild-type enzyme was 90% homologous with the aliphatic amidase from Pseudomonas aeruginosa, and Southern blotting using an oligonucleotide probe for this region showed that overexpression of the enzyme in the mutant strains was not due to gene amplification. Compared with the wild-type and MM6 enzymes, the MM8 enzyme exhibited a threefold higher K m and a slightly lower K m for acrylamide, whereas the MM15 enzyme exhibited a similar K cat and an eightfold lower K m for acrylamide. The MM15 enzyme also reacted more extensively with the thiol group reagent DTNB, had a significantly lower sedimentation coefficient and exhibited a more relaxed substrate specificity, all of which were compatible with a looser tetrameric structure. It was also much more susceptible than the other three enzymes to inactivation by high temperature or by freezing and thawing (MM15»MM8>MM6/wild-type), both of which variably dissociated the enzyme into inactive dimers and monomers. The amidase activity of strain MM15 was almost 15-fold higher following growth at 25 °C than at 37 °C, since at this lower temperature the enzyme exhibited a similar K cat to the MM8 enzyme and was not significantly dissociated. However, as strain MM15 readily outgrew the organism from which it was derived (strain MM8) during acrylamide-limited continuous culture at 37 °C, it is clear that under these conditions a low K m was a greater selective advantage than a high K cat.