Biosynthesis of nucleic acids in Bacillus megaterium. 4. Roles of the ‘nuclear’ cytoplasmic and cytoplasmic-membrane components of the cell in the biosynthesis of ribonucleic acid

Abstract

When protoplasts of B. megaterium are incubated for short periods with [H3] uridine, ribo-nucleic-acid (RNA) with the highest specific radioactivity is present in the "nuclear"material and cytoplasmic membrane. The "nuclear" ribosomes (possibly attached to the cytoplasmic membrane) have a higher specific radioactivity than the cytoplasmic ribosomes, but this difference diminishes with time, suggesting a precursor relationship. Labelling of the RNA in all cell fractions (except the cytoplasmic fraction) is quickly inhibited by actinomycin-D. The inhibition of the labelling of the membrane RNA occurs more slowly than that of the "nuclear" fractions. There is no apparent physical difference between the "nuclear" and cytoplasmic ribosomes. The higher specific radioactivity of the "nuclear" ribosomes is due to the presence of more labelled ribosomal and messenger RNA. The "nuclear" ribosomes may contain the initial products of nuclear RNA synthesis. Only the 16s RNA of the 30s ribosome subunit is labelled initially, but the 23 s RNA of the 50s ribosome subunit is labelled later. The cytoplasmic membrane is an important site of protein synthesis in the cell.