Methionine biosynthesis in Agrobacterium tumefaciens: study of the first enzyme

Here we characterize the first step in methionine biosynthesis in Agrobacterium tumefaciens, an α-proteobacterium. We explored the metA gene and its products and found several unique properties. Although the gene was annotated as a homoserine transsuccinylase, based upon sequence similarity to characterized homologs in other bacteria, including Escherichia coli, the enzyme uses acetyl-CoA as a substrate and therefore is functionally a transacetylase. Moreover, the protein is thermolabile and the gene is under regulation of heat shock transcriptional activator σ32. 3. The gene has a SAM-riboswitch, which shuts off transcription by σ-32 as well as by the vegetative σ-70.