Chloramphenicol inhibits bacterial protein synthesis by binding the 50S ribosomal subunit. Chloramphenicol is effective against both Gram (+) and (-) bacteria and some mycobacteria. Resistance to chloramphenicol is conferred by the cat gene which encodes an acetyltransferase that acetylates and inactivates the antibiotic. Chloramphenicol is commonly used for the amplification of vectors in Gram (-) bacteria.