POSTTRANSCRIPTIONAL REGULATION OF PROTEIN-SYNTHESIS DURING ALFALFA EMBRYOGENESIS - PROTEINS ASSOCIATED WITH THE CYTOPLASMIC POLYSOMAL AND NONPOLYSOMAL MESSENGER-RNAS (MESSENGER-RIBONUCLEOPROTEIN COMPLEX)

Cytoplasmic polysomal and non-polysomal mRNA-associated protein comple xes were isolated from, and characterized in, developing somatic and z ygotic embryos of alfalfa (Medicago sativa L.). S-35-methionine-labell ed intact embryos were irradiated with ultraviolet light (UV) in situ to cross-link mRNA and proteins occurring within one bond length, and the polysomal and non-polysomal fractions were extracted. Then the mRN A-protein complexes were isolated from the fractions and separated usi ng two cycles of affinity chromatography on an oligo(dT)-cellulose col umn. Following digestion with RNase A and T1 and micrococcal nuclease, mRNA-associated proteins were separated by sos-PAGE. Several polypept ides of 15-150 kDa were associated with the polysomal and non-polysoma l (ribonucleoprotein, mRNP) fractions of alfalfa embryos after UV-cros s-linking. Many of the polypeptides associated with the polysomal and non-polysomal mRNAs were qualitatively similar, although their concent ration in the two fractions was different. However, some developmental ly stage-specific polypeptides were found to be associated with the no n-polysomal mRNA fraction during the early stages of embryogenesis (pr e-cotyledonary) of somatic embryos. Thus the presence of mRNPs during embryogenesis has been demonstrated, and proteins intimately associate d with the mRNAs identified.