Influence of systematic error on the shape of the scatchard plot of tRNAPhe binding to eukaryotic ribosomes

Scatchard plots of tRNAPhe binding to poly(U)-programmed human 80 S ribosomes can be curved, either concave upwards or concave downwards, depending on the experimental conditions. The influence of a systematic error on the shape of the Scatchard plots has been analysed in a model experiment where the binding proceeds at two independent sites. The Scatchard plot for this binding model has a concave-upwards shape. When the concentration of the ribosomes is kept constant, a small systematic error in tRNA concentration changes this Scatchard plot markedly to a concave-downwards plot as though a co-operative interaction occurred. In contrast, when the tRNA concentration exceeds the ribosomal concentration and their concentration ratio is constant, the Scatchard plot is stable with respect to the systematic error. We suggest the latter type of experiment to be more appropriate. The results also imply a non-co-operative interaction of tRNAPhe with the 80 S ribosome.