Characteristics of ribosomes isolated from roots of Zea mays

Ribosomes were isolated from root tips of corn (Zea mays) and their properties studied. Five ribosomal components (40 S, 60 S, 80 S, 120 S, and 150 S), with the 80-S component predominating, were present in the cytoplasmic extract. Sedimentation coefficients at infinite dilution were determined for all except the 150-S component.

Purified ribosomes contained 43% RNA which was characterized by a high guanine and relatively high cytosine content. The 80-S component dissociated into 60-S and 40-S particles when pH of the medium was increased from 6.5 to 8.0 or higher. Magnesium (0.5–2 mM) prevented the dissociation.

In marked contrast to the 70-S ribosome of Escherichia coli, the 80-S ribosome of corn did not require added magnesium to maintain its integrity in a solution of nearly neutral pH and low ionic strength. In fact, 5 mM magnesium in such a solution caused the association of corn ribosomes into large random aggregates. Ribosomes of different organisms differ in the level of magnesium required to preserve their 70–80-S component and may be divided accordingly into three broad groups exemplified by E. coli, yeast, and corn ribosomes.