In several organisms, developmental transitions are accompaniedby transfer RNA (tRNA) alterations. These alterations are usuallyobserved in the chromatographic profile of amino acyl-tRNA specificfor one or more amino acids and are of interest because of their possiblesignificance in the regulation, at the translation level, ofspecific protein synthesis and cell differentiation. I have investigatedwhether such alterations accompany the biochemical differentiation ofvegetative cultures of Neurspora crassa which occurs in response to"hard-times," e.g., starvation or inhibition by amino acid analogs orcycloheximide. The synthesis of tyrosinase is a well-known characteristicof this developmental transition.
After determining the conditions required for the completecharging of all 20 amino acids to Neurospora tRNA, I compared thechromatographic profile on methylated albumin-Kieselguhr columns ofamino acyl-tRNA's from vegetative cultures to those of cultures whichwere derepressed for tyrosinase with ethionine, a methionine analog.No qualitative tRNA alterations were observed; the same number ofcomponents for each amino acid were found in cultures of both developmentalstates and they had the same chromatographic mobilities.However, quantitative changes of acceptor activity were observed forseveral amino acids. The time course of the pattern of quantitativealteration suggests that the observed changes result from partial ribonucleasedigestion of the tRNA complement. I believe this ribonucleaseis synthesized in response to the deprived environment and its functionis to hydrolyze the RNA which is present in excess, in order that thecatabolic products may be used as building blocks for the synthesisof other kinds of molecules.
