Polyamine stimulation and cation requirements of rabbit liver tRNA nucleotidyltransferase

Purification and characterization of the Escherichia coli exoribonuclease RNase R. Comparison with RNase II

Escherichia coli RNase R, a 3' --> 5' exoribonuclease homologous to RNase II, was overexpressed and purified to near homogeneity in its native untagged form by a rapid procedure. The purified enzyme was free of nucleic acid. It migrated upon gel filtration chromatography as a monomer with an apparent molecular mass of approximately 95 kDa, in close agreement with its expected size based on the sequence of the rnr gene. RNase R was most active at pH 7.5-9.5 in the presence of 0.1-0.5 mm Mg(2+) and 50-500 mm KCl. The enzyme shares many catalytic properties with RNase II. Both enzymes are nonspecific processive ribonucleases that release 5'-nucleotide monophosphates and leave a short undigested oligonucleotide core. However, whereas RNase R shortens RNA processively to di- and trinucleotides, RNase II becomes more distributive when the length of the substrate reaches approximately 10 nucleotides, and it leaves an undigested core of 3-5 nucleotides. Both enzymes work on substrates with a 3'-phosphate group. RNase R and RNase II are most active on synthetic homopolymers such as poly(A), but their substrate specificities differ. RNase II is more active on poly(A), whereas RNase R is much more active on rRNAs. Neither RNase R nor RNase II can degrade a complete RNA-RNA or DNA-RNA hybrid or one with a 4-nucleotide 3'-RNA overhang. RNase R differs from RNase II in that it cannot digest DNA oligomers and is not inhibited by such molecules, suggesting that it does not bind DNA. Although the in vivo function of RNase R is not known, its ability to digest certain natural RNAs may explain why it is maintained in E. coli together with RNase II.

Ornithine decarboxylase may be a multifunctional protein

Ornithine decarboxylase may undergo posttranslational modifications which alter its function. Both transamidation of glutamine residues in the enzyme catalyzed by TGase and phosphorylation of serine and threonine residues catalyzed by a polyamine-stimulated protein kinase have been demonstrated. Data are presented which suggest that these modifications result in translocation of the modified protein to the nucleolus where it regulates the activity of RNA polymerase I to transcribe rDNA, the only active nucleolar genes. Transamidation of specific proteins with primary amines catalyzed by intracellular TGase may be an important posttranslational modification, capable of altering genetic transcription. The rapid half-life of ODC (10-15 min) may be related to rapid posttranslational modification with loss of enzymatic activity rather than to protein degradation.

Association of spermine and 4S RNA during axonal transport in regenerating optic nerves of goldfish

Experiments were designed to determine whether polyamines are bound to 4S RNA and then transported axonally along regenerating optic axons of goldfish. In one set of experiments, inhibition of retinal RNA synthesis by intraocular injections of 10 microgram of cordycepin, blocked the axonal transport of both [3H]RNA and [14C]spermidine by about 65%, 6 and 14 days after injection. Intraocular injections of vinblastine, (0.1, 0.5 or 1.0 microgram) an agent which interrupts axonal transport of proteins, had no effect on retinal RNA synthesis nor on the amount of [14C]spermidine incorporated into the TCA-insoluble fraction of retinal extracts. However, the axonal transport of both [3H]RNA and [14C]polyamines was affected in a dose-dependent fashion; the inhibition of both was approximately 80% at the higher dose. Further evidence for an association between axonally transported 4S RNA and polyamines came from experiments in which regenerating optic axons were cut and allowed to degenerate 6 days after injection of [3H]spermidine into the eye. The loss of optic axons from the tectum 7 days after cutting the nerve resulted in an 86% loss of TCA insoluble polyamines, indicating a largely intra-axonal locus. A similar loss of 4S RNA was found in identical experiments following injections of [3H]uridine into the eye. Finally, experiments were performed in which [3H]spermidine was injected into both eyes of 12 fish whose optic nerves had been regenerating for 18 days. Six days later, fish were sacrificed and RNA was extracted from tectal homogenates by hot phenol and ethanol precipitation. The major stable RNA species were separated by SDS-polyacrylamide disc gel electrophoresis and radioactivity was determined by extraction of 2.0 mm gel slices. Results showed co-migration of 3H with 4S RNA optical density peaks, and not with 28S and 18S ribosomal RNA peaks, suggesting that some polyamine-associated radioactivity is bound to axonally transported 4S RNA. When the nature of that radioactivity was determined on an amino acid analyzer, it was found to be present primarily as spermine and not as the injected compound spermidine. The data are consistent with the hypothesis that some spermine is bound to 4S RNA and then axonally transported along regenerating axons of the goldfish optic nerve.

Purification and characterization of a mutant tRNA nucleotidyltransferase

tRNA nucleotidyltransferase has been extensively purified from a mutant strain of Escherichia coli which displays greatly decreased AMP incorporation, but normal CMP incorporation. The defect in AMP incorporation is retained throughout the purification of the mutant protein. The mutant protein behaves identically to the wild-type protein with regard to elution position on various chromatographic columns, and both have similar molecular weights of about 50000. The defect in the mutant protein is accentuated by the use of yeast tRNA rather than E. coli tRNA-C--C as substrate, by decreased pH, by increased ionic strength and by decreased divalent cation concentration. Substitution of MN2+ for Mg2+ greatly increases the relative activity of the mutant enzyme. In all these cases, CMP incorporation by the mutant enzyme remains the same as the wild-type enzyme. The Km values of the mutant enzyme for its tRNA and triphosphate substrates are unchanged, and the mutant protein is as stable as the wild type with respect to temperature inactivation. These results strongly suggest that the mutation is in the structural gene for tRNA nucleotidyltransferase, and that the mutation probably does not affect the overall structure of the mutant protein, but only a localized region near the AMP-incorporating site.

Plant tRNA nucleotidyltransferase : II. Some properties of the purified enzyme from Lupinus luteus seeds

Purified lupine tRNA nucleotidyltransferase catalyzed only the incorporation of AMP and CMP into 3'-terminal-C-C-A sequences of tRNA as determined by terminal analysis of the reaction product. The incorporation of AMP and CMP was inhibited by their respective triphosphates to different degrees; UTP also showed an inhibitory effect. The pH optimum of the purified enzyme was found to be 9.5 in glycine buffer. The enzyme required magnesium or manganese ions for its activity.-SH reagents reversibly inhibited the action of the enzyme. Kinetic data, the different effects of ionic strength on the incorporation of AMP and CMP, and different rates of thermal inactivation for AMP and CMP incorporation in connection with chromatographic properties of the enzyme suggest the existence of only one form of tRNA nucleotidyltransferase with different catalytic sites for ATP and CTP.