The function of the histidine tRNA isoaccepting species in hemoglobin synthesis

Rabbit reticulocytes contain two RNA isoaccepting species for histidine as resolved by various chromatographic methods, while rabbit liver contains only one. These isoacceptors cannot be distinguished on the basis of coding properties, consistent with the "Wobble Hypothesis" (Smith, D.W.E., Meltzer, V.N., and McNamara, A.L. (1974) Biochim. Biophys. Acta 349, 366-375). Their function in hemoglobin synthesis in reticulocyte lysates has been investigated. Each of the tRNA isoacceptors of reticulocytes and the tRNA species of liver can incorporate histidine into positions in hemoglobin encoded by both of the histidine code words, CAC and CAU, and it is likely that each can incorporate histidine into all of the histidine-containing positions of hemoglobin. Even in experiments in which the two histidine tRNA species of reticulocytes are placed together in a lysate and are therefore in competition with each other, each incorporates histidine into all of the histidine-containing positions. There is no evidence that any residues are incorporated preferentially by either of the tRNA species. The two species are attached to reticulocyte ribosomes in the same proportion as they occur in the reticulocyte, also suggesting that neither of them is used preferentially in hemoglobin synthesis. The first of the two reticulocyte histidine isoacceptors and the histidine tRNA of rabbit liver contain Q base.