Is an L-glutamate ligase that catalyzes the ATP-dependent post-translational addition of glutamate residues to the C-terminus of ribosomal protein S6 (RpsF). Is also able to catalyze the synthesis of poly-alpha-glutamate in vitro, via ATP hydrolysis from unprotected glutamate as substrate. The number of glutamate residues added to either RpsF or to poly-alpha-glutamate changes with pH. Binds 2 magnesium or manganese ions per subunit. Optimum pH is 9.5 for RpsF modification and 9 for polyglutamate synthase activity. Shows thermal stability. Exhibits 86% activity after incubation at 55 degrees Celsius for 15 minutes, but its activity decreases sharply at 60 degrees Celsius. This protein may find application in fermentative methods that use microorganisms overexpressing rimK for mass production of poly-alpha-amino acids, which is thought to be the most economical and ecofriendly manufacturing process. Belongs to the RimK family.