All other components of the reaction were not radioactive. Since only ribosome-bound tRNA is retained by the nylon membrane, all radioactivity found on the nylon membrane corresponds to tRNA bound to ribosomes. Now a system was in place to detect the recognition between a mRNA molecule and the proper amino-acylated tRNA. To test his system, Nirenberg used polyU as the mRNA, and [14C]phenylalanine-charged tRNA, which binds to ribosomes in the presence of polyU.