Corynebacterium glutamicumUtilizes both Transsulfuration and Direct Sulfhydrylation Pathways for Methionine Biosynthesis

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Abstract
A direct sulfhydrylation pathway for methionine biosynthesis inCorynebacterium glutamicumwas found. The pathway was catalyzed bymetYencodingO-acetylhomoserine sulfhydrylase. The genemetY, located immediately upstream ofmetA, was found to encode a protein of 437 amino acids with a deduced molecular mass of 46,751 Da. In accordance with DNA and protein sequence data, the introduction ofmetYintoC. glutamicumresulted in the accumulation of a 47-kDa protein in the cells and a 30-fold increase inO-acetylhomoserine sulfhydrylase activity, showing the efficient expression of the cloned gene. Although disruption of themetBgene, which encodes cystathionine γ-synthase catalyzing the transsulfuration pathway of methionine biosynthesis, or themetYgene was not enough to lead to methionine auxotrophy, an additional mutation in themetYor themetBgene resulted in methionine auxotrophy. The growth pattern of themetYmutant strain was identical to that of themetBmutant strain, suggesting that both methionine biosynthetic pathways function equally well. In addition, anEscherichia coli metBmutant could be complemented by transformation of the strain with a DNA fragment carrying corynebacterialmetYandmetAgenes. These data clearly show thatC. glutamicumutilizes both transsulfuration and direct sulfhydrylation pathways for methionine biosynthesis. AlthoughmetYandmetAare in close proximity to one another, separated by 143 bp on the chromosome, deletion analysis suggests that they are expressed independently. As withmetA, methionine could also repress the expression ofmetY. The repression was also observed withmetB, but the degree of repression was more severe withmetY,which shows almost complete repression at 0.5 mM methionine in minimal medium. The data suggest a physiologically distinctive role of the direct sulfhydrylation pathway inC. glutamicum.

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