Har Gobind Khorana at the University of Wisconsin Synthesized Artificial

Har Gobind Khorana at the University of Wisconsin synthesized artificial mRNAs such as poly CA (CACA … ) and poly CAA (CAACAACAA … ).He found that poly CA codes for a polypeptide consisting of alternating threonine (Thr) and histidine (His) residues.There are two possible codons in poly CA: CAC and ACA.One of these must encode histidine and the other threonine-but which is which? The answer comes from results with poly CAA, which produces three different polypeptides: poly Thr, poly Gln (glutamine), and poly Asn (asparagine).(An artificial mRNA can be read, inefficiently, beginning at any point in the chain; there is no specific initiation signal.Thus poly CAA can be read as a polymer of CAA, of ACA, or of AAC.) Compare the results of the poly CA and poly CAA experiments, and determine which codon corresponds with threonine and which with histidine.