In mammals methionine adenosyltransferase (MAT), the enzyme responsible for S-adenosylmethionine (AdoMet) synthesis, is encoded by two genes, MAT1A and MAT2A. In liver, MAT1A expression is associated with high AdoMet levels and a differentiated phenotype, whereas MAT2A expression is associated with lower AdoMet levels and a dedifferentiated phenotype. In the current study we examined regulation of MAT2A gene expression by L-methionine availability using HepG2 cells. In L-methionine-deficient cells MAT2A gene expression is rapidly induced, and methionine adenosyltransferase activity is increased. Restoration of L-methionine rapidly down-regulates MAT2A mRNA levels; for this effect L-methionine needs to be converted into AdoMet. This novel action of AdoMet is not mediated through a methyl-transfer reaction. MAT2A gene expression was also regulated by 5’-methylthioadenosine (MTA), but this was dependent on MTA conversion to methionine through the salvage pathway. The transcription rate of MAT2A gene remained unchanged during L-methionine starvation, however its mRNA half-life was significantly increased (from 100 min. to more than 3 h). The effect of L-methionine withdrawal on MAT2A mRNA stabilization requires both gene transcription and protein synthesis. We conclude that MAT2A gene expression is modulated as an adaptive response of the cell to L-methionine availability through its conversion to AdoMet.