Production of l -Phenylalanine from Starch by Analog-Resistant Mutants of Bacillus polymyxa

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Abstract
p -Fluorophenylalanine-resistant mutants of starch-degrading Bacillus polymyxa ATCC 842, generated by ethyl methanesulfonate mutagenesis followed by incubation with caffeine, overproduced small amounts of l -phenylalanine ( l -phe) from starch. A β-2-thienylalanine-resistant mutant (BT R -7) derived from p -fluorophenylalanine mutant (C-4000 FP R -4) and resistant to both p -fluorophenylalanine and β-2-thienylalanine produced 0.5 g of l -phe and 0.15 g of l -tyrosine per liter from 10 g of starch per liter when growing in a minimal medium. trans -Cinnamic acid (CA) was also excreted by both mutants, indicating the possibility of l -phenylalanine ammonia-lyase-induced deamination of l -phe to CA. The amount of l -phe-derived CA detected in BT R -7 was less compared with mutant C-4000 FP R -4. CA production was induced in the parent only when l -phe was used as a sole nitrogen source. Time of CA production in the two mutants could be delayed by addition of other nitrogen sources, an indication of possible l -phenylalanine ammonia-lyase inhibition or repression. The presence of l -phenylalanine ammonia-lyase in B. polymyxa mutant C-4000 FP R -4 was confirmed by assays of cell-free extracts from cells grown in starch minimal medium containing l -phe as the sole nitrogen source. Preliminary studies of the regulation of deoxy- d -arabino-heptulosonate-7-phosphate synthase and prephenate dehydratase in the wild-type strain showed that deoxy- d -arabino-heptulosonate-7-phosphate synthase was subject to feedback inhibition by l -phe, l -tyrosine, and l -tryptophan. Inhibition by each amino acid was to a similar extent singly or in combination at a 0.5 mM level of each amino acid. Prephenate dehydratase was feedback inhibited by l -phe, but not by l -tyrosine or l -tryptophan or both. In the double analog-resistant mutant BT R -7, deoxy- d -arabino-heptulosonate-7-phosphate synthase had specific activity similar to that in the wild type, and the enzyme was still subject to feedback inhibition. However, prephenate dehydratase had increased specific activity and it was also insensitive to feedback inhibition by l -phe. The overproduction of aromatic amino acids by BT R -7 was thought to be due, at least in part, to deregulation of feedback inhibition of prephenate dehydratase. Chorismate mutase was not subject to feedback inhibition in the wild type and was unaffected in the mutant.