Protein synthesis and the viability of rye grains. Loss of activity of protein-synthesizing systems in vitro associated with a loss of viability

A study was made of the integrity of some components of the protein-synthesizing system from viable and non-viable embryos of rye grains. In comparison with viable-embryo components both post-ribosomal supernatant and ribosomal fractions from non-viable embryos are impaired, for neither will fully support polyphenylalanine synthesis in poly(U)-directed cell-free systems. The lesion in the supernatant lies in components other than the tRNA or the aminoacyl-tRNA synthetase, for these are as functional as those present in the fully active cell-free systems from viable embryos. The ribosomes of embryos of lowered viability show considerable fragmentation and degradation of both 18S and 25S rRNA. This breakdown does not, however, account for the complete lack of polypeptide synthesis in the poly(U)-directed non-viable-embryo system, for if provided with viable-embryo supernatant, non-viable-embryo ribosomes will sustain 60% of the viable-embryo ribosome activity. A lesion in non-viable-embryo supernatant has been located in the binding of the aminoacyl-tRNA to the ribosome. The impaired components in both supernatant and ribosomes in systems in vitro may reflect the site of faults in protein synthesis in vivo in the early hours of germination. The development of these lesions during grain storage could contribute to senescence and loss of viability in the embryos of rye.

Nucleocytoplasmic Interaction in the Eggs of Pteridium Aquilinum Maturing in the Presence of Thiouracil

In the eggs of Pteridium aquilinum maturing in the presence of a small quantity of thiouracil (0.6 x 10-3 M in the medium), known to reduce the rate of maturation by a half or more, the nuclear evaginations are particularly conspicuous. On the grounds of their structural features, their regularly narrow connexion with the nucleus, and the absence of nuclear pores from their envelopes, the evaginations are regarded as specialized formations, and not merely extensions of an amoeboid nucleus. An aggregate of electron-opaque material, probably of nucleolar origin, and believed on the basis of enzyme extractions to be largely protein, but with some ribonucleic acid, is a characteristic feature of the evaginations.

The fate of the evaginations, which disappear shortly after fertilization, remains obscure. Refined methods of fixation and staining have, however, revealed striking resemblances between the evaginations, bodies which may be detached evaginations, and mitochondria. The hypothesis that the evaginations become detached, and that some at least differentiate into mitochondria, the simplest interpretation of the events observed, has not been invalidated by this investigation.

Capsella Embryogenesis: the Chalazal Proliferating Tissue

Electron-microscope and histochemical procedures were used to study the development and breakdown of the chalazal proliferating tissue in Capsella. This tissue is formed by the enlargement of several layers of nucellar cells at the chalazal end of the embryo sac. When the embryo reaches the early globular stage these enlarged cells start to disintegrate, beginning with those immediately bordering the embryo sac and continuing until all have broken down. Characteristic ultrastructural changes accompany the development and breakdown of the chalazal proliferating cells. The mature cells form increased numbers of dictyosomes and large amounts of endoplasmic reticulum (ER). This is accompanied by a thickening of the cell wall. As the cells begin to break down, electron density increases, plastids become polarized in the cells, dictyosomes disappear and the ER is dispersed and fragmented. Plastids, some mitochondria, and pieces of ER appear to be digested in autophagic vacuoles. Cell disorganization is accompanied by an increased number of microbodies and multivesicular bodies per cell. Finally, the nucleus breaks down and the plasmalemma disappears. The end wall ruptures and releases intact mitochondria, ribosomes, and portions of degenerated cytoplasm into the endosperm. Histochemical changes accompany these events. Also discussed are the antipodals and the destruction of the proximal part of the chalazal nucellus by the expanding megagametophyte prior to the development of the chalazal proliferating tissue.

In vitro O 2 '-methylation of sugars in E. coli RNA. 3. Methylation of E. coli transfer RNA by heterologous methylases in particulate-free extracts of wheat embryo

(1) Transfer ribonucleates (tRNA) from a relaxed strain of E. coli have been methylated by homologous enzymes from E. coli and by heterologous enzymes from wheat embryo, using particulate-free extracts supplemented with [methyl-14C]-S-adenosylmethionine.(2) For both homologous and heterologous systems, the methylation of bases greatly exceeded the methylation of sugars, although the patterns of base and sugar methylation were radically different for homologous and heterologous methylations. Thus, the primary targets for homologous methylation were pyrimidine nucleosides: ribothymidine, the primary product of base methylation, and O2′-methylcytidine in the sequence CmpU, the primary product of sugar methylation. On the other hand, the primary targets for heterologous methylation were purine nucleosides: N2,N2-dimethylguanosine, the primary product of base methylation, and O2′-methylguanosine in the sequence GmpG, the primary product of sugar methylation.(3) The results of an allied study of the methylation of wheat embryo tRNA by homologous wheat embryo methylases and heterologous E. coli methylases have also been reported.