The answer is P-site. Ribosomes are composed of two subunits, one
small and one large. Four binding sites are located on the ribosome, one for
mRNA and three for tRNA. The three tRNA sites are labeled P, A, and E. The A
site (acceptor site), binds to the aminoacyl tRNA, which holds the new amino
acid to be added to the polypeptide chain.
