Preformed messenger of quiescent wheat embryos

Abstract

A subcellular fraction (messenger fraction) which supports in vitro amino acid incorporation, has been isolated from dry wheat embryos. Analyses of the characteristics of messenger fraction directed amino acid incorporation (kinetics, nucleotide requirements, K + and Mg 2+ optima, and response to inhibitors) indicate that a ribosome-messenger attachment reaction is a prerequisite for messenger fraction activity. Direct demonstration of this reaction is provided by experiments in which messenger fraction is allowed to react in vitro with messenger-free ribosomes. Sucrose density gradient analysis of the reaction shows a progressive conversion of monoribosomes to functional polyribosomes. Upon storage at o° the activity of messenger fraction decays with a half-life of 12–13 h. Loss of activity is not due to ribonuclease as shown by complete stability of tobacco mosaic virus (TMV) RNA stored in the presence of messenger fraction. These data as well as experiments demonstrating the sensitivity of messenger fraction to low concentrations of pronase and N-ethyl maleimide suggest that a component other than RNA is necessary for messenger fraction activity and that the functional form of messenger fraction may be a ribonucleoprotein. The low level of template activity of purified messenger fraction RNA is consistent with this conclusion. In vivo studies show a rapid decrease in messenger fraction concomitant with polyribosome formation during early germination. These data suggest an important physiological role for messenger fraction in the initiation of protein synthesis which occurs at the outset of germination.