Acidi Nucleici E Maturazione dell'Endosperma del seme di Ricino: II. Variazioni a carico delle diverse frazioni di RNA

Abstract

Nucleic Acids in Ripening Castor Bean Seed Endosperm. - II. Changes of the RNA from Different Fractions. — Quantitative and qualitative changes of the RNA from Castor Bean seed endosperm (Ricinus Communis var. Sanguinea) have been studied at two ripening stages (maximal fresh weight seed and dry seed).

The seeds were homogenized in 0,05 M. Tris-HCl pH 7.6 buffer, 0.004 M Mg Acetate, 0.025 M KCl, 0.005 M mercaptoethanol, 0.3 M sucrose, and the following fractions were separated by centrifugation:

1. a « nuclear » fraction (0–600 × g); b) a « mitochondrial » fraction 600–13,000 × g), c) a « ribosomal » fraction (13,000–105,000 × g); d) a « soluble » fraction (supernatant of 105,000 × g).

The behaviour of the RNA purified from the different fractions by phenol estraction (H. L. Sanger, C. A. Knight, Biochem. Biophys. Res, Comm., 13, 445, 1963) was the following:

1. The RNA of the « nuclear » fraction decreases from about 330 μg per seed in the first stage, to 170 μg in the dry seed.

2. The RNA of the « mitochondrial » fraction, initially 180 μg per seed, practically disappears.

3. The RNA of the « ribosomal » fraction decreases from 160 μg to 20 μg.

4. The RNA of the « soluble » fraction considerably increases during ripening. This rise however is not large enough to compensate for the decrease of the other fractions.

Ribosome behaviour has been furtherly studied by sucrose density gradient franctionation of the 13,000 × g supernatant, according to A. Monroy and A. Tyler (Arch. Biochem. Biophys., 103, 431, 1963).

This experiment too has shown that the ribosomes, present in the early stage of developement, completely disappear during the ripening of the seed.

These data show that the RNA decrease during the ripening process is mostly important in the particulate fractions of the cytoplasm (nuclei, mitochondria, ribosomes): this could be an important factor of the progressive metabolic and enzymatic inactivation that characterizes the ripening of the seed.

The increase of the low molecular weight RNA suggests that during the ripening of the endosperm an enhancement of RNA degradation occurs. Thus the decrease of heavy RNA during ripening seems to be due to both the inactivation of RNA synthesis and the acceleration of its breakdown.