Cystathionine as a precursor of methionine in Escherichia coli and Aerobacter aerogenes

Betaine-homocysteine transmethylase in Pseudomonas denitrificans, a vitamin B 12 overproducer

A pantothenate-methionine auxotroph (J741) of Pseudomonas denitrificans was isolated whose growth requirement for methionine could not be satisfied by known precursors of the amino acid, including homocysteine. However, some "methyl rich" compounds such as betaine and dimethylacetothetin (DMT) could satisfy the requirement. S-Methyl-methionine and S-adenosylmethionine were ineffective. Extracts were found to contain an enzyme, betaine-homocysteine transmethylase (BHTase), that uses betaine or DMT as a methyl donor and homocysteine as an acceptor to produce methionine. Growth of J741 in methionine leads to a total repression of the BHTase, whereas the use of DMT leads to a three- to sixfold stimulation of enzyme synthesis compared to betaine-grown cells. The pantothenate requirement is unrelated to the methionine auxotrophy, since the growth of other single auxotrophic mutants as well as revertants of J741 still have their methionine requirement satisfied by betaine or DMT. Another methionine auxotroph that could not use betaine for growth was devoid of BHTase activity.

Cystathionine metabolism in methionine auxotrophic and wild-type strains of Saccharomyces cerevisiae

The role of cystathionine in methionine biosynthesis in wild-type and auxotrophic strains of Saccharomyces cerevisiae was studied. Homocysteine and cysteinerequiring mutants were selected for detailed study. Exogenously supplied cystathionine, although actively transported by all strains tested, could not satisfy the organic sulfur requirements of the mutants. Cell-free extracts of the wild-type, homocysteine, and cysteine auxotrophs were shown to cleave cystathionine. Pyruvic acid and homocysteine were identified as teh products of this cleavage. A mutant containing an enzyme which could cleave cystathionine to homocysteine in cell-free experiments was unable to use cystathionine as a methionine precursor in the intact organisms. The significance of this finding is discussed.

Inhibition of growth of Escherichia coli and of homoserine O-transsuccinylase by alpha-methylmethionine

The methionine analogue, alpha-methylmethionine, inhibits bacterial growth, but its action is overcome by methionine, homocysteine, and cystathionine. The effect of the analogue on growth is attributed to its ability to mimic methionine as a feed-back inhibitor of the first enzyme specific to methionine biosynthesis. This conclusion is based on the findings that (i) alpha-methylmethionine inhibits excretion of O-succinylhomoserine, the product of the first enzyme, by a methionine auxotroph unable to convert succinylhomoserine to cystahionine, and that (ii) the enzyme homoserine O-transsuccinylase is inhibited by alpha-methylmethionine in extracts of Escherichia coli. alpha-Methylmethionine also inhibits methionyl-ribonucleic acid synthetase in extracts, but this inhibition probably does not affect growth.