It uses activated amino acids in the form of aminoacyl-tRNAs (aa-tRNAs) as substrates to catalyze the ATP-independent formation of cyclodipeptides which are intermediates in diketopiperazine (DKP) biosynthetic pathways. Catalyzes the formation of cyclo(L-Leu-L-Leu) (cLL) from L-leucyl-tRNA(Leu). Can incorporate various nonpolar residues, such as L-leucine and L-methionine, into cyclodipeptides