Processing of naked 45-S ribosomal RNA precursor in vitro by an RNA-associated endoribonuclease

Processing of eukaryotic ribosomal RNA

In summary, it can be argued that the understanding of eukaryotic rRNA processing is no less important than the understanding of mRNA maturation, since the capacity of a cell to carry out protein synthesis is controlled, in part, by the abundance of ribosomes. Processing of pre-rRNA is highly regulated, involving many cellular components acting either alone or as part of a complex. Some of these components are directly involved in the modification and cleavage of the precursor rRNA, while others direct the packaging of the rRNA into ribosome subunits. As is the case for pre-mRNA processing, snoRNPs are clearly involved in eukaryotic rRNA processing, and have been proposed to assemble with other proteins into at least one complex called a "processosome" (17), which carries out the ordered processing of the pre-rRNA and its assembly into ribosomes. The formation of a processing complex clearly makes possible the regulation required to coordinate the abundance of ribosomes with the physiological and developmental changes of a cell. It may be that eukaryotic rRNA processing is even more complex than pre-mRNA maturation, since pre-rRNA undergoes extensive nucleotide modification and is assembled into a complex structure called the ribosome. Undoubtedly, features of the eukaryotic rRNA-processing pathway have been conserved evolutionarily, and the genetic approach available in yeast research (6) should provide considerable knowledge that will be useful for other investigators working with higher eukaryotic systems. Interestingly, it was originally hoped that the extensive work and understanding of bacterial ribosome formation would provide a useful paradigm for the process in eukaryotes. However, although general features of ribosome structure and function are highly conserved between bacterial and eukaryotic systems, the basic strategy in ribosome biogenesis seems to be, for the most part, distinctly different. Thus, the detailed molecular mechanisms for rRNA processing in each kingdom will have to be independently deciphered in order to elucidate the features and regulation of this important process for cell survival.

The activity of neutral ribonucleases in nuclei of rat sympathetic ganglia and effects of nerve injury

Nuclei were isolated from homogenates of rat superior cervical ganglion by a conventional differential centrifugation technique with approximately 60% recovery. Ribonuclease activity at pH 7.1 (neutral ribonuclease) was associated with the "nuclei fraction" and represented 19% of the overall activity in normal ganglia. Ribonuclease in the "nuclei fraction" was stimulated variably by the sulfhydryl blocker N-ethylmaleimide indicating that a proportion was bound to the endogenous ribonuclease inhibitor present in these ganglia. The total activity of nuclear ribonuclease was increased 2-6 days after postganglionic nerve injury, such that the inhibitor-bound form of the enzyme increased maximally by 600% at day 4. The percentage of the total ganglionic activity in the "nuclei fraction" decreased in injured ganglia as a result of a rise in the activity of non-nuclear components. The changes in nuclear ribonuclease activity were distinct from those in the 850 g supernatant indicating that specific nuclear enzymes are being affected during regeneration.

Ribonuclease activities in rat sympathetic ganglia: evidence for the presence of an endogenous inhibitor of alkaline ribonuclease

Using 3H-labeled rat brain mature RNA as substrate, substantial ribonuclease activity was detected in homogenates of rat superior cervical ganglia with acidic (pH 5.5) and neutral (pH 7.0-7.5) optima. Very little activity could be measured at greater than pH 8. The acidic and neutral activities differed in the optimal conditions required for assay, and showed differential sensitivity to the sulfhydryl blocking agent, N-ethylmaleimide. Only the neutral activity was stimulated, optimally by 2 mM N-ethylmaleimide, and the magnitude of stimulation indicated that the contributing ribonucleases exist largely in a latent form in the ganglion. Ribonucleases in other tissues with neutral pH dependence, known usually as "alkaline" ribonucleases, are subject to an N-ethylmaleimide-sensitive endogenous inhibitor protein. The existence of a similar inhibitor in rat superior cervical ganglia was indicated by the latency of neutral ribonuclease activity and confirmed by observing the effect of a soluble fraction from the ganglia on the activity of pancreatic ribonuclease A.

Double-stranded RNA specific nuclease from germinating embryos of Pennisetum typhoides

A double-stranded RNA specific nuclease (ds RNase) has been purified from the pearl millet Pennisetum typhoides. The purification involved S-30 preparation from the germinating embryos, DEAE-cellulose and DNA-cellulose chromatography. The partially pure enzyme preferentially solubilized the synthetic double-stranded polynucleotide [3H]poly(rA) . poly(rU); the degradation of [3H]poly(rC) was fourteen fold lower under the same assay conditions. Furthermore, the ds RNase activity was inhibited to an extent of 58% by ethidium bromide, which is known to intercalate with double-stranded RNAs. Active sulfhydryl groups were found to be necessary for the ds RNase activity since the enzyme action was inhibited by N-ethylmaleimide. Ethidium bromide and N-ethyl-maleimide did not significantly inhibit the ss RNase activity. In contrast, diethyl pyrocarbonate inhibited ss RNase activity completely and ds RNase by 58%. Heating the enzyme for 20 min at 50 degrees C resulted in drastic loss of both enzyme activities. The ds RNase showed maximum activity in the pH range of 6.5 to 7.5. The enzyme acts in vitro on E. coli 30S precursor ribosomal RNA and the cleavage products migrated in the region of mature 23S and 16S rRNAs.