Katabolism of plant cytoplasmic ribosomes: A study of the interaction between ribosomes and ribonuclease

Identification, characterization, and comparison of RNA-degrading enzymes of wheat and barley

RNA-degrading enzymes play an important role in regulating gene expression, and sequence analyses have revealed significant homology among several plant RNA-degrading enzymes. In this study we surveyed crude extracts of the above-ground part of the common wheat (Triticum aestivum L.) and the cultivated barley (Hordeum vulgare L.) for major RNA-degrading enzymes using a substrate-based SDS-PAGE assay. Fifteen wheat and fourteen barley RNA-degrading enzymes, with apparent molecular masses ranging from 16.3 to 40.1 kD, were identified. These RNA-degrading enzymes were characterized by their response to pH changes and addition of EDTA and ZnCl2 to the preincubation or incubation buffers. The 33.2- to 40.1-kD wheat and barley, 31.7-kD wheat, and 32.0-kD barley enzyme activities were inhibited by both zinc and EDTA and were relatively tolerant to alkaline environment. The 22.7- to 28.2-kD enzymes were inhibited by zinc but stimulated by EDTA. The 18.8-kD enzyme exists in both wheat and barley. It was active in an acid environment, was inhibited by zinc, but was not affected by EDTA. Two enzyme activities (31.0 and 32.0 kD) are unique to the common wheat.

Preservation of algal and higher plant ribosomal RNA integrity during extraction and electrophoretic quantitation

Isolation of high molecular weight ribosomal RNA from the wall-less alga Olisthodiscus luteus and the angiospermous plant Sauromatum guttatum is described. It has been found that a buffer which contains magnesium must be used to successfully isolate Olisthodiscus rRNA whereas the isolation of intact Sauromatum rRNA requires a buffer system containing a high amount of the chelator EDTA. Sauromatum but not Olisthodiscus extracts were contaminated with ribonuclease unless the inhibitor diethylpyrocarbonate was used during the ribonucleic acid extraction procedure. Nuclease levels were monitored by coincubating [3H]-labeled Escherichia coli ribosomal RNA with the experimental RNA samples. The effects of detergents on the isolation and quantitation of RNA are presented, and methods to avoid loss of highly thermolabile plant ribosomal RNA species are discussed.

Ribosomes in the lutoïd fraction (=lysosomal compartment) from Hevea brasiliensis Künth. (Mull.-arg.) latex

Lutoïd ribosomes represent 11.9% of the total ribosomal content of latex from Hevea brasilensis. Their two high-molecular-weight RNA components are identified. When undegraded, their molecular weights are calculated to be 1.3×10(6) and 0.7×10(6). Their nucleotide base composition is determined. The occurrence of lutoïd membrane-bound ribosomes and the lability of the 1.3×10(6)RNA component are discussed in relation to the biological role of lutoïds, more generally of a lysosomal compartment in the cellular function.

Examination of ribosome-like particles in isolated prolamellar bodies

Potential methods for the preparation of fractions enriched in prolamellar bodies (PLBs) were examined in detail. Sucrose density gradient centrifugation methods gave fractions consisting almost exclusively of PLBs whilst those methods employing differential centrifugation were quite successful but contained greater quantities of lamellar membranes. Greater difficulty was experienced in obtaining detached PLBs which retained their "ribosome-like" lattice particles. No modification to density gradient procedures was found which retained these particles but the omission of ethylene diaminetetraacetic acid (EDTA) from all media including that of lysis gave a hint that this was possible with differential centrifugal methods. This was developed to produce a successful method for the preparation of PLBs which retain the "ribosome-like" particles of the lattice. Such fractions from Avena sativa L. and Hordeum vulgare L. were treated with ribonuclease which completely removed these particles from the lattice structures implying that they may be "ribosomal" in nature. EDTA apparently has a critical effect on PLB structure at concentration lower than those that effect the chloroplast coupling factor particles but it is not known if it is a direct effort of PLB membranes, on the lattice particles or both.

Particle-bound phytochrome: The nature of the interaction between pigment and particulate fractions

Particle-bound phytochrome from hypocotyl hooks of Cucurbita pepo L. seedlings is associated with either a "heavy" membrane fraction or a 31S ribonucleoprotein (RNP) component on sucrose gradients. Those amounts of phytochrome and RNP which co-pellet in response to added Mg(2+) are primarily localised in the 31S fraction. The phytochrome-RNP association itself, however, is not dependent on the added cation. This indicates that Mg(2+)-enhanced phytochrome pelletability results indirectly from aggregation of the RNP material. Phytochrome binds readily to this RNP fraction whether converted to the Pfr form in vivo or in vitro. Once bound, in either case, however, the pigment is not released by reconversion to Pr in vitro. Treatment of pellets with Triton X-100 causes most of the phytochrome to become sedimentable through 50% (w/w) sucrose, possibly indicative of pigment denaturation. Increasing the pH, in contrast, causes that phytochrome formerly located in the "heavy" membrane fraction to become associated with the 31S RNP component. High KCl concentrations dissociate the pigment from both "heavy" and 31S fractions, indicating the ionic nature of the interaction in both cases. These data can be accounted for by the electrostatic adsorption of phytochrome to ribosomal material, either ER-associated in the "heavy" fraction or "free" in the 31S fraction.Maize (Zea mays L.) exhibits a different pattern. Although Mg(2+) enhances the initial pelletability of both phytochrome and RNA the two components subsequently separate on Mg(2+)-free gradients.The data indicate that current interpretations of particle-bound phytochrome in terms of pigment-membrane interactions may need to be re-examined.

Particle-bound phytochrome: Association with a ribonucleoprotein fraction from Cucurbita pepo L

In the absence of ethylenediaminetetraacetic acid (EDTA) and added Mg(2+), the phytochrome, RNA, protein, cytochrome c oxidase and NADPH-cytochrome c reductase in 20000 x g pellets from hypocotyl hooks of red-irradiated Cucurbita seedlings are more or less coincident in a single, broad band on linear sucrose gradients. The inclusion of 3 mM EDTA in the extraction, resuspension and gradient media has three major effects: (a) The phytochrome profile splits into two main bands; (b) the main RNA population shifts to a sharp peak which co-sediments with the "lighter" phytochrome band at 31S; (c) the main NADPH-cytochrome c reductase peak shifts to a lower density. This indicates that the EDTA dissociates a rough-endoplasmic-reticulum fraction into separate membrane and ribonucleoprotein (RNP) components, and that part of the phytochrome is associated with the latter. The 31S RNP fraction is 35-40% RNA, has a 260/235 nm absorption ratio of 1.36 and the RNA dissociates into small fragments in sodium dodecyl sulfate. More than 90% of the phytochrome and RNA in the isolated 31S fraction becomes pelletable upon the addition of 10 mM Mg(2+). Higher Mg(2+) levels release the phytochrome and some of the other protein present from the RNA which remains pelletable. The data indicate that the 31S RNP fraction may be degraded ribosomal material with extraneously bound protein, including phytochrome. Several aspects of phytochrome-binding to particulate fractions which have been reported in the literature are consistent with an interaction of Pfr with ribosomal material-degraded or otherwise.

Katabolism of plant cytoplasmic ribosomes: RNA breakdown in senescent cotyledons of germinating broad-bean seedlings

Broad-bean cotyledons lost about 80% of their RNA during the first 30 days of seed germination. Both high-molecular-weight ribosomal (r) RNA and low-molecular-weight RNA were degraded at about the same rate. Only small quantities of breakdown products of rRNA were detected within senescent tissue. This finding contrasts strongly with the very rapid cleavage of rRNA and the accumulation of breakdown products when the same cotyledons were homogenised and then incubated at 25°. It would appear, therefore, that there was a barrier between ribonuclease and most ribosomes in vivo which was disrupted by homogenisation. The results are discussed in relation to the mechanism of ribosome katabolism during senescence.