Involved in the biosynthesis of albonoursin (cyclo[(alpha,beta-dehydro-Phe)-(alpha,beta-dehydro-Leu)]), an antibacterial peptide. 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-Phe-L-Leu) (cFL) as major products from L-L-phenylalanyl-tRNA(Phe) and L-leucyl-tRNA(Leu). AlbC can also incorporate various nonpolar residues, such as L-phenylalanine, L-leucine, L-tyrosine and L-methionine, and to a much lesser extent L-alanine and L-valine, into cyclodipeptides. Indeed, ten possible cyclodipeptides composed of L-phenylalanine, L-leucine, L-tyrosine and L-methionine are all synthesized to detectable amounts by AlbC. L-leucyl-tRNA(Leu) + L-phenylalanyl-tRNA(Phe) = cyclo(L-phenylalanyl-L-leucyl) + H(+) + tRNA(Leu) + tRNA(Phe) Monomer. The reaction proceeds following a ping-pong mechanism forming a covalent intermediate between an active site Ser-37 and the L-phenylalanine residue. Belongs to the CDPS family.