Changes in polysomes in Nicotiana glutinosa L. leaves infected with lettuce necrotic yellows virus

Abstract

Polysomes have been studied in extracts of leaves of Nicotiana glutinosa L. infected systemically with lettuce necrotic yellows virus (LNYV). Before symptoms appeared, at the time of rapid virus synthesis, no changes in polysome concentration or size distribution were observed compared with healthy leaves. However, in infected leaves treated with actinomycin D (AMD) and labelled with 32P, increased ribonuclease-sensitive radioactivity was observed in the fractions of sucrose gradients containing polysomes, and three labelled nucleic acids were isolated from the fractions containing monosomes and polysomes. Uninfected, control leaves showed very much lower ribonuclease-sensitive radioactivity in the fractions containing polysomes, and the monosome-polysome fractions contained only one of the three radioactive nucleic acids detected in infected leaves. It is suggested that the radioactive nucleic acids found in infected leaves treated with AMD could be associated with LNYV-specific polysomes. Commencing with the appearance of symptoms, extracts of infected leaves showed a decrease in the proportion of ribosomes occurring as polysomes, compared with healthy leaves. As symptoms developed, the proportion of ribosomes occurring as polysomes continued to decline more rapidly than in healthy leaves and this was accompanied by a net loss of ribosomes. Within a day of the appearance of symptoms the ability of infected leaves to incorporate [ 14C]valine into protein had declined compared with healthy leaves. This suggested that symptom production in LNYV-infected N. glutinosa was accompanied by a reduced rate of protein synthesis. An observed general increase in the content of soluble amino acids, amines and peptides in infected leaves indicated that the reduced rate of protein synthesis was not due to a decrease in the size of the amino acid pool.