Sedimentation properties of preformed messenger particles in dry rye embryo extracts

Reproducible preparation of a wheat germ cell-free system

This study has examined whether the conditions used for the precipitation of plant mRNPs are suitable for the reduction of endogenous template activity in the wheat germ cell-free system. These results have led to a modification of the procedure for the preparation of the wheat germ system that allows the consistent preparation of an active in vitro translation system with low endogenous activity.

Stored messenger ribonucleoprotein particles in differentiated sclerotia of Physarum polycephalum

Starvation induces vegetative microplasmodia of Physarum polycephalum to differentiate into translationally-dormant sclerotia. The existence and the biochemical nature of stored mRNA in sclerotia is examined in this report. The sclerotia contain about 50% of the poly (A)-containing RNA [poly(A)+RNA] complement of microplasmodia as determined by [3H]-poly(U) hybridization. The sclerotial poly(A)+RNA sequences are associated with proteins in a ribonucleoprotein complex [poly(A)+mRNP] which sediments more slowly than the polysomes. Sclerotial poly(A)+RNP sediments more rapidly than poly(A)+RNP derived from the polysomes of microplasmodia despite the occurrence of poly(A)+RNA molecules of a similar size in both particles suggesting the existence of differences in protein composition. Isolation of poly(A)+RNP by oligo (dT)-cellulose chromatography and the analysis of its associated proteins by polyacrylamide gel electrophoresis show that sclerotial poly(A)+RNP contains at least 14 major polypeptides, 11 of which are different in electrophoretic mobility from the polypeptides found in polysomal poly(A)+RNP. Three of the sclerotial poly(A)+RNP polypeptides are associated with the poly(A) sequence (18, 46, and 52 x 10(3) mol. wt. components), while the remaining eight are presumably bound to non-poly(A) portions of the poly(A)+RNA. Although distinct from polysomal poly(A)+RNP, the sclerotial poly(A)+RNP is similar in sedimentation behavior and protein composition (with two exceptions) to the microplasmodial free cytoplasmic poly(A)+RNP. The results suggest that dormant sclerotia store mRNA sequences in association with a distinct set of proteins and that these proteins are similar to those associated with the free cytoplasmic poly(A)+RNP of vegetative plasmodia.

Botanical aspects of cell-free protein synthesizing systems from cereal embryos

Some botanical aspects of cell-free protein synthesizing systems from cereal embryos have been investigated. The composition of the starting material determines both the stability and translation fidelity of the cell-free extracts. The active components of the extracts originate exclusively from the primary axes. Contamination with scutellum fragments does not affect the initial activity but results in a reduced stability. The presence of endosperm particles in the starting material leads to a strong decrease of the overall activity of the extracts and a loss of the capacity to synthesize large polypeptides.

Protein synthesis in imbibing wheat embryos

Polypeptides synthesized by imbibing wheat embryos have been compared with those made by cell-free extracts programmed with bulk poly(A)-rich RNA from dry wheat embryos. Newly synthesized polypeptides, labeled with [35S]methionine, were resolved by one-dimensional and two-dimensional electrophoresis and then records of the separations were prepared by fluorography. When programmed by bulk poly(A)-rich RNA from dry wheat embryos, a nuclease-treated rabbit reticulocyte lysate synthesizes an array of polypeptides which is broadly similar to that formed when a wheat germ extract is programmed with the same RNA. Polypeptides made in both homologous and heterologous cell-free systems, under the direction of bulk poly(A)-rich RNA from dry wheat embryos, are broadly similar to those formed during early (0--40 min) imbibition of dry wheat embryos. As imbibition progresses beyond 40 min, there are profound changes in the one-dimensional and two-dimensional electrophoretic distributions of newly made polypeptides present in the 23 000 x g supernatant fraction of cell-free homogenates; characteristically, low-molecular-weight and basic polypeptides comprise a diminishing proportion of the total polypeptides as imbibition progresses beyond 40 min. Ribosomal proteins are conspicuous among the proteins formed during early imbibition and especially prominent among the products formed when homologous cell-free polypeptide synthesis is programmed by bulk poly(A)-rich RNA from dry wheat embryos.

Preformed and newly synthesized messenger RNA in germinating wheat embryos

In 6 h germinated wheat (Triticum aestivum L. cv. Cama) embryos, more than half of the messenger RNAs are actively involved in translation. Neither preformed nor newly synthesized poly A(+)-RNA is translated preferentially. Germination in the presence of cordycepin showed that the half-life of the templates is about 2 h and that the newly synthesized messengers are essential to support protein synthesis in the embryo from the first hours of germination. Most of the messenger RNAs in 6 h germinated embryos are newly synthesized. The polypeptides coded for by either the endogenous messenger ribonucleoproteins or purified poly A(+)-RNA from both dry and germinated embryos are qualitatively identical; minor quantitative differences can however be observed.

Some aspects of the synthesis of long-lived messenger ribonucleoproteins in developing rye embryos

Untranslated messenger ribonucleoproteins (mRNPs informosomes) are present in developing rye (Secale cereale, L. cv. Celestijner) embryos throughout the last 5 weeks of seed formation. Ribosomal as well as non-ribosomal ribonucleoproteins are formed continuously both in primary axes and scutella until any synthesis of macromolecules stops upon dessication. The content of preformed messengers in the primary axes of precociously harvested rye grains increases as a function of embryo development. This increase of the "template load" of the primary axes results from a continuous accumulation of qualitatively identical mRNPs. Germination experiments demonstrated that at least most of the preformed messengers are not required for the germination process. Their function is discussed in terms of selective adaptation to unfavorable conditions.