3' processing of tRNA precursors in ribonuclease-deficient Escherichia coli. Development and characterization of an in vitro processing system and evidence for a phosphate requirement

Bacterial ribonucleases and their roles in RNA metabolism

Ribonucleases (RNases) are mediators in most reactions of RNA metabolism. In recent years, there has been a surge of new information about RNases and the roles they play in cell physiology. In this review, a detailed description of bacterial RNases is presented, focusing primarily on those from Escherichia coli and Bacillus subtilis, the model Gram-negative and Gram-positive organisms, from which most of our current knowledge has been derived. Information from other organisms is also included, where relevant. In an extensive catalog of the known bacterial RNases, their structure, mechanism of action, physiological roles, genetics, and possible regulation are described. The RNase complement of E. coli and B. subtilis is compared, emphasizing the similarities, but especially the differences, between the two. Included are figures showing the three major RNA metabolic pathways in E. coli and B. subtilis and highlighting specific steps in each of the pathways catalyzed by the different RNases. This compilation of the currently available knowledge about bacterial RNases will be a useful tool for workers in the RNA field and for others interested in learning about this area.

Exoribonucleases and Endoribonucleases

This review provides a description of the known Escherichia coli ribonucleases (RNases), focusing on their structures, catalytic properties, genes, physiological roles, and possible regulation. Currently, eight E. coli exoribonucleases are known. These are RNases II, R, D, T, PH, BN, polynucleotide phosphorylase (PNPase), and oligoribonuclease (ORNase). Based on sequence analysis and catalytic properties, the eight exoribonucleases have been grouped into four families. These are the RNR family, including RNase II and RNase R; the DEDD family, including RNase D, RNase T, and ORNase; the RBN family, consisting of RNase BN; and the PDX family, including PNPase and RNase PH. Seven well-characterized endoribonucleases are known in E. coli. These are RNases I, III, P, E, G, HI, and HII. Homologues to most of these enzymes are also present in Salmonella. Most of the endoribonucleases cleave RNA in the presence of divalent cations, producing fragments with 3'-hydroxyl and 5'-phosphate termini. RNase H selectively hydrolyzes the RNA strand of RNA?DNA hybrids. Members of the RNase H family are widely distributed among prokaryotic and eukaryotic organisms in three distinct lineages, RNases HI, HII, and HIII. It is likely that E. coli contains additional endoribonucleases that have not yet been characterized. First of all, endonucleolytic activities are needed for certain known processes that cannot be attributed to any of the known enzymes. Second, homologues of known endoribonucleases are present in E. coli. Third, endonucleolytic activities have been observed in cell extracts that have different properties from known enzymes.

The PNPase, exosome and RNA helicases as the building components of evolutionarily-conserved RNA degradation machines

The structure and function of polynucleotide phosphorylase (PNPase) and the exosome, as well as their associated RNA-helicases proteins, are described in the light of recent studies. The picture raised is of an evolutionarily conserved RNA-degradation machine which exonucleolytically degrades RNA from 3' to 5'. In prokaryotes and in eukaryotic organelles, a trimeric complex of PNPase forms a circular doughnut-shaped structure, in which the phosphorolysis catalytic sites are buried inside the barrel-shaped complex, while the RNA binding domains create a pore where RNA enters, reminiscent of the protein degrading complex, the proteasome. In some archaea and in the eukaryotes, several different proteins form a similar circle-shaped complex, the exosome, that is responsible for 3' to 5' exonucleolytic degradation of RNA as part of the processing, quality control, and general RNA degradation process. Both PNPase in prokaryotes and the exosome in eukaryotes are found in association with protein complexes that notably include RNA helicase.

Probing the functional importance of the hexameric ring structure of RNase PH

RNase PH is a phosphate-dependent exoribonuclease that catalyzes the removal of nucleotides at the 3' end of the tRNA precursor, leading to the release of nucleoside diphosphate, and generates the CCA end during the maturation process. The 1.9-A crystal structures of the apo and the phosphate-bound forms of RNase PH from Pseudomonas aeruginosa reveal a monomeric RNase PH with an alpha/beta-fold tightly associated into a hexameric ring structure in the form of a trimer of dimers. A five ion pair network, Glu-63-Arg-74-Asp-116-Arg-77-Asp-118 and an ion-pair Glu-26-Arg-69 that are positioned symmetrically in the trimerization interface play critical roles in the formation of a hexameric ring. Single or double mutations of Arg-69, Arg-74, or Arg-77 in these ion pairs leads to the dissociation of the RNase PH hexamer into dimers without perturbing the overall monomeric structure. The dissociated RNase PH dimer completely lost its binding affinity and catalytic activity against a precursor tRNA. Our structural and mutational analyses of RNase PH demonstrate that the hexameric ring formation is a critical feature for the function of members of the RNase PH family.

Exoribonucleases and their multiple roles in RNA metabolism

In recent years there has been a dramatic shift in our thinking about ribonucleases (RNases). Although they were once considered to be nonspecific, degradative enzymes, it is now clear that RNases play a central role in every aspect of cellular RNA metabolism, including decay of mRNA, conversion of RNA precursors to their mature forms, and end-turnover of certain RNAs. Recognition of the importance of this class of enzymes has led to an explosion of work and the establishment of significant new concepts. Thus, we now realize that RNases, both endoribonucleases and exoribonucleases, can be highly specific for particular sequences or structures. It has also become apparent that a single cell can contain a large number of distinct RNases, approaching as many as 20 members, often with overlapping specificities. Some RNases also have been found to be components of supramolecular complexes and to function in concert with other enzymes to carry out their role in RNA metabolism. This review focuses on the exoribonucleases, both prokaryotic and eukaryotic, and details their structure, catalytic properties, and physiological function.

RNA processing in prokaryotic cells

RNA processing in Escherichia coli and some of its phages is reviewed here, with primary emphasis on rRNA and tRNA processing. Three enzymes, RNase III, RNase E and RNase P are responsible for most of the primary endonucleolytic RNA processing events. The first two are proteins, while RNase P is a ribozyme. These three enzymes have unique functions and in their absence, the cleavage events they catalyze are not performed. On the other hand a relatively large number of exonucleases participate in the trimming of the 3' ends of tRNA precursor molecules and they can substitute for each other. Primary processing is the first event that happens to the nascent RNA molecule, while in secondary RNA processing, the substrate is a product of a primary processing event. Although most RNA processing occurs in RNP particles, it seems that only in secondary RNA processing is the RNP particle required for the reaction. Bacteria and especially bacteriophages contain self-splicing introns which in cases were probably acquired from other species.

Identification of the rph (RNase PH) gene of Bacillus subtilis: evidence for suppression of cold-sensitive mutations in Escherichia coli

A shotgun cloning of Bacillus subtilis DNA into pBR322 yielded a 2-kb fragment that suppresses the cold-sensitive defect of the nusA10(Cs) Escherichia coli mutant. The responsible gene encodes an open reading frame that is greater than 50% identical at the amino acid level to the E. coli rph gene, which was formerly called orfE. This B. subtilis gene is located at 251 degrees adjacent to the gerM gene on the B. subtilis genetic map. It has been named rph because, like its E. coli analog, it encodes a phosphate-dependent exoribonuclease activity, RNase PH, that removes the 3' nucleotides from precursor tRNAs. The cloned B. subtilis rph gene also suppresses the cold-sensitive phenotype of other unrelated cold-sensitive mutants of E. coli, but not the temperature-sensitive phenotype of three temperature-sensitive mutants, including the nusA11(Ts) mutant, that were tested.

Escherichia coli orfE (upstream of pyrE) encodes RNase PH

RNase PH from extracts of Escherichia coli was purified to homogeneity and subjected to NH2-terminal sequencing. Comparison of this sequence with all open reading frames in the GenBank data base revealed at least 95% identity to an unidentified open reading frame (orfE) upstream of pyrE at 81.7 min on the E. coli chromosome. Clones of orfE overexpress RNase PH activity, verifying that orfE encodes this ribonuclease. We suggest that orfE be renamed rph.

RNase PH catalyzes a synthetic reaction, the addition of nucleotides to the 3' end of RNA

Escherichia coli RNase PH is a phosphate-dependent exoribonuclease that has been implicated in the 3' processing of tRNA precursors. It degrades RNA chains in a phosphorolytic manner releasing nucleoside diphosphates as products. Here we show that RNase PH also catalyzes a synthetic reaction, the addition of nucleotides to the 3' termini of RNA molecules. The synthetic activity co-purifies with RNase PH throughout an extensive enrichment indicating that it is due to the same enzyme. The synthetic activity can incorporate all nucleoside diphosphates, but not triphosphates, and is strongly inhibited by Pi, but not PPi. Various RNA molecules stimulate nucleotide incorporation, and with tRNA the 3' end of the molecule serves a primer function. RNA chains as long as 40 residues can be synthesized in this system. As with polynucleotide phosphorylase, the synthetic activity of RNase PH apparently represents the reversal of the degradative reaction.

Processing of transcripts of a dimeric tRNA gene in yeast uses the nuclease responsible for maturation of the 3' termini upon 5 S and 37 S precursor rRNAs

The rna82 mutation of Saccharomyces cerevisiae inactivates an RNA processing activity responsible for maturation of 3'-terminal sequences upon 5 S and 37 S ribosomal RNA precursors. This study describes a difference in the processing of transcripts of an S. cerevisiae dimeric tRNA gene (tRNA(arg)-tRNA(Asp) in RNA polymerase III in vitro transcription extracts prepared from rna82 and wild-type cells. The mutant extract accumulated additional processing intermediates containing tRNA(Arg) sequences as compared to the extract from wild-type cells. The structure of these intermediates revealed a defect in removal of the 10 nucleotides left 3' to the tRNA(Arg) sequence by the RNase P cleavage immediately 5' to tRNA(Asp). This is the first demonstration of a mutational defect affecting maturation of 3' sequences upon a eukaryotic tRNA precursor.