Involved in the conversion of folates to polyglutamate derivatives, and likely functions in the retention of cellular folate pools. Catalyzes successive MgATP-dependent additions of glutamate to a pteroylmonoglutamate substrate, with a high preference for 5,10-methylenetetrahydrofolate (mTHF). Thus, metabolizes mTHF to the tetraglutamate derivative, but longer glutamate chain length products are not observed. Tetrahydrofolate (H4PteGlu) and 10-formyl-H4PteGlu are poorer folate substrates. In contrast to E.coli FolC, this enzyme does not display dihydrofolate synthase activity. (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 (6R)-5,10-methylenetetrahydrofolyl-(gamma-L-Glu)(n) + ATP + L-glutamate = (6R)-5,10-methylenetetrahydrofolyl-(gamma-L-Glu)(n+1) + ADP + H(+) + phosphate 10-formyltetrahydrofolyl-(gamma-L-Glu)(n) + ATP + L-glutamate = 10-formyltetrahydrofolyl-(gamma-L-Glu)(n+1) + ADP + H(+) + phosphate Binds 2 Mg(2+) ions per subunit. Competitively inhibited by adenosine 5'-(3-thio)triphosphate and beta,gamma-methylene-ATP. Optimum pH is about 10. Monomer. In contrast to many bacteria such as E.coli and Corynebacterium spp., L.casei cannot synthesize folate de novo, and requires exogenous folates for growth. L.casei metabolizes folate to polyglutamates of chain length up to 11, with octa- and nonaglutamates predominating. Kinetic studies are consistent with an ordered Ter-Ter mechanism with MgATP binding first to the enzyme, folate second, and glutamate last. The order of product dissociation from the enzyme is ADP, folate product, and Pi. Belongs to the folylpolyglutamate synthase family.