Maureen E. Callow

In vitro protein synthesis, ribosomal RNA synthesis, and polyribosomes in senescing leaves of Perilla

Abstract Chloroplasts from growing leaves of Perilla were active in the incorporation of labelled glycine into polypeptide, but chloroplasts from mature and senescent leaves were less active, or inactive. Active cytoplasmic ribosomes were, however, obtained from both growing and senescent leaves. Fractionation of ribosomes on sucrose gradients indicated that the in vitro activities were approximately correlated with the polyribosome contents of the leaves. Cytoplasmic (80S-based) polyribosomes were present even in older leaves, whereas chloroplast (70S-based) polyribosomes could only be isolated in quantity from growing leaves. Chloroplast rRNA appears to be synthesized only in growing leaves but there appears to be a continued turnover of cytoplasmic rRNA until late in the life of the leaf.