Catalyzes conversion of folates to polyglutamate derivatives allowing concentration of folate compounds in the cell and the intracellular retention of these cofactors, which are important substrates for most of the folate-dependent enzymes that are involved in one-carbon transfer reactions involved in purine, pyrimidine and amino acid synthesis. Required for methionine synthesis and maintenance of intact mitochondrial DNA. Involved in telomere maintenance. (6S)-5,6,7,8-tetrahydrofolyl-(gamma-L-Glu)(n) + ATP + L-glutamate = (6S)-5,6,7,8-tetrahydrofolyl-(gamma-L-Glu)(n+1) + ADP + H(+) + phosphate A monovalent cation. Cofactor biosynthesis; tetrahydrofolylpolyglutamate biosynthesis. Methionine auxotroph. Undetectable levels of folylpolyglutamate synthase activity. Increased rate of loss of mitochondrial DNA. Respiration-deficient. MET7 and FOL3 double mutant cells are not viable even in the presence of methionine and folinic acid. MET7, FOL3 and TUP1 triple disruption mutant is able to germinate on YPGA medium containing thymidylate and to grow although poorly on synthetic medium containing methionine, adenine and thymidylate. Adenine and thymidine auxotroph when grown in the presence of sulfanilamide. Becomes petite under normal growth conditions but can be maintained with a grande phenotype if the strain is TUP1 and all media are supplemented with dTMP. MET7 and GLY1 double mutant is auxotrophic for glycine when grown on glucose but prototrophic when grown on glycerol. MET7 and SER1 double mutant cannot use glycine to suppress serine auxotrophy. MET7 and SHM2 double mutant is nonviable. Present with 7080 molecules/cell in log phase SD medium. Belongs to the folylpolyglutamate synthase family.