Polyadenylated mRNA from the photosynthetic procaryote Rhodospirillum rubrum

Bacterial/archaeal/organellar polyadenylation

Although the first poly(A) polymerase (PAP) was discovered in Escherichia coli in 1962, the study of polyadenylation in bacteria was largely ignored for the next 30 years. However, with the identification of the structural gene for E. coli PAP I in 1992, it became possible to analyze polyadenylation using both biochemical and genetic approaches. Subsequently, it has been shown that polyadenylation plays a multifunctional role in prokaryotic RNA metabolism. Although the bulk of our current understanding of prokaryotic polyadenylation comes from studies on E. coli, recent limited experiments with Cyanobacteria, organelles, and Archaea have widened our view on the diversity, complexity, and universality of the polyadenylation process. For example, the identification of polynucleotide phosphorylase (PNPase), a reversible phosphorolytic enzyme that is highly conserved in bacteria, as an additional PAP in E. coli caught everyone by surprise. In fact, PNPase has now been shown to be the source of post-transcriptional RNA modifications in a wide range of cells of prokaryotic origin including those that lack a eubacterial PAP homolog. Accordingly, the past few years have witnessed increased interest in the mechanism and role of post-transcriptional modifications in all species of prokaryotic origin. However, the fact that many of the poly(A) tails are very short and unstable as well as the presence of polynucleotide tails has posed significant technical challenges to the scientific community trying to unravel the mystery of polyadenylation in prokaryotes. This review discusses the current state of knowledge regarding polyadenylation and its functions in bacteria, organelles, and Archaea.

Polyadenylation of mRNA in prokaryotes

The 3'-ends of both prokaryotic and eukaryotic mRNA are polyadenylated, but the poly(A) tracts of prokaryotic mRNA are generally shorter, ranging from 15 to 60 adenylate residues and associated with only 2-60% of the molecules of a given mRNA species. The sites of polyadenylation of bacterial mRNA are diverse and include the 3'-ends of primary transcripts, the sites of endonucleolytic processing in the 3' untranslated and intercistronic regions, and sites within the coding regions of mRNA degradation products. The diversity of polyadenylation sites suggests that mRNA polyadenylation in prokaryotes is a relatively indiscriminate process that can occur at all mRNA's 3'-ends and does not require specific consensus sequences as in eukaryotes. Two poly(A) polymerases have been identified in Escherichia coli. They are encoded by unlinked genes, neither of which is essential for growth, suggesting significant functional overlap. Polyadenylation promotes the degradation of a regulatory RNA that inhibits the replication of bacterial plasmids and may play a similar role in the degradation of mRNA. However, under certain conditions, poly(A) tracts may lead to mRNA stabilization. Their ability to bind S1 ribosomal protein suggests that poly(A) tracts may also play a role in mRNA translation.

Regulation of ribulose bisphosphate carboxylase expression in Rhodospirillum rubrum: characteristics of mRNA synthesized in vivo and in vitro

The synthesis of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase) in Rhodospirillum rubrum was regulated by the CO2 concentration in the culture medium. The specific activity of RuBPCase in cells grown photolithotrophically in low concentrations of CO2 (1.5%) was five to ten times higher than that in cultures grown at high concentrations of CO2 (10%). Increased enzyme activity was reflected by an increase in both RuBPCase mRNA and RuBPCase protein. RuBPCase expression was also studied in vitro with a plasmid-borne genomic clone (pRR117) as the template in a partially defined Escherichia coli system containing either E. coli or R. rubrum RNA polymerase. With both enzymes there was excellent synthesis of RuBPCase mRNA, but no significant synthesis of RuBPCase was detected. The promoter region of the RuBPCase gene was sequenced, and mRNA start sites were mapped. A single major in vivo transcriptional start site was detected in RuBPCase mRNA extracted from R. rubrum. However, transcripts synthesized from pRR117 in vitro or from E. coli transformed with pRR117 started at upstream sites that were different from the in vivo transcription site. Two major features of the RuBPCase promoter region are three 6-base-pair direct repeats and a 31-base-pair region of dyad symmetry.

3'-terminal polyadenylate sequences of Escherichia coli tryptophan synthetase alpha-subunit messenger RNA

Our earlier studies have shown that the mRNA from many bacterial species, including Escherichia coli and Bacillus subtilis, is extensively polyadenylated, but with shorter poly(A) segments than those associated with eukaryotic mRNA. In this paper, we show that about 40% of the mRNA for the tryptophan synthetase alpha-subunit (TrpA) of E. coli carries a 3'-terminal polyadenylate sequence of 15 to 20 residues. This conclusion was supported by several independent lines of evidence. About 40% of trpA mRNA bound to oligo(dT)-cellulose at high ionic strength and was eluted with water. Treatment with RNase H in the presence of oligo(dT)12-18 destroyed the ability of trpA mRNA to bind to oligo(dT)-cellulose, presumably through the degradation of the poly(A) tract. trpA mRNA could be used as template for complementary DNA synthesis with reverse transcriptase in a reaction that was absolutely dependent on oligo(dT)12-18 as primer. The identity of the cDNA product as a complement to trpA mRNA was established by specific hybridization. In addition, it was possible to synthesize polyadenylated trpA mRNA in toluene-permeabilized cells of E. coli transformed with a recombinant plasmid carrying the trpA gene. In view of the fact that the trpA gene and its 3'-untranslated region contain no continuous deoxyadenylate sequences larger than five nucleotides, one can conclude that the polyadenylate moiety is added post-transcriptionally.

Messenger ribonucleic acid for the lipoprotein of the Escherichia coli outer membrane is polyadenylated

Earlier studies had shown that a large portion of bacterial messenger RNA carries 3'-terminal polyadenylate sequences, albeit of somewhat shorter length than those associated with eukaryotic mRNA. In this paper, we show for the first time that a specific prokaryotic mRNA is polyadenylated. Three independent lines of evidence demonstrate that a 3'-terminal polyadenylate sequence 10 to 15 nucleotides in length is associated with about 40% of the mRNA of the outer membrane lipoprotein of Escherichia coli: 40% of lipoprotein mRNA binds to oligodeoxythymidylate-substituted cellulose at high ionic strength and is eluted by water; treatment of lipoprotein mRNA with oligodeoxythymidylate and ribonuclease H destroys its ability to bind to oligodeoxythymidylate-cellulose; and in the presence of oligodeoxythymidylate, lipoprotein mRNA can serve as template for the synthesis of DNA complementary to lipoprotein mRNA by reverse transcriptase. In view of the fact that the lpp gene and its downstream-flanking region contain no continuous deoxyadenylate sequences longer than five nucleotides, the polyadenylate moiety must be added post-transcriptionally. It was possible to demonstrate the synthesis of polyadenylated lipoprotein mRNA in toluene-permeabilized cells of E. coli, opening the way for the study of its biosynthesis.

Polyadenylated RNA isolated from the archaebacterium Halobacterium halobium

Polyadenylated [poly(A)+] RNA has been isolated from the halophilic archaebacterium Halobacterium halobium by binding, at 4 degrees C, to oligo(dT)-cellulose. H. halobium contains approximately 12 times more poly(A) per unit of RNA than does the methanogenic archaebacterium Methanococcus vannielii. The 3' poly(A) tracts in poly(A)+ RNA molecules are approximately twice as long (average length of 20 nucleotides) in H. halobium as in M. vannielii. In both archaebacterial species, poly(A)+ RNAs are unstable.

Polyadenylated, noncapped RNA from the archaebacterium Methanococcus vannielii

Polyadenylated [poly(A)+] RNA molecules have been isolated from Methanococcus vannielii by oligodeoxythymidylate-cellulose affinity chromatography at 4 degrees C. Approximately 16% of the label in RNA isolated from cultures allowed to incorporate [3H]uridine for 3 min at 37 degrees C was poly(A)+ RNA. In contrast, less than 1% of the radioactivity in RNA labeled over a period of several generations was contained in poly(A)+ RNA molecules. Electrophoretic separation of poly(A)+ RNA molecules showed a heterogeneous population with mobilities indicative of sizes ranging from 900 to 3,000 bases in length. The population of poly(A)+ RNA molecules was found to have a half-life in vivo of approximately 12 min. Polyadenylate [poly(A)] tracts were isolated by digestion with RNase A and RNase T1 after 3' end labeling of the poly(A)+ RNA with RNA ligase. These radioactively labeled poly(A) oligonucleotides were shown by electrophoresis through DNA sequencing gels to average 10 bases in length, with major components of 5, 9, 10, 11, and 12 bases. The lengths of these poly(A) sequences are in agreement with estimates obtained from RNase A and RNase T1 digestions of [3H]adenine-labeled poly(A)+ RNA molecules. Poly(A)+ RNA molecules from M. vannielii were labeled at their 5' termini with T4 polynucleotide kinase after dephosphorylation with calf intestine alkaline phosphatase. Pretreatment of the RNA molecules with tobacco acid pyrophosphatase did not increase the amount of phosphate incorporated into poly(A)+ RNA molecules by polynucleotide kinase, indicating that the poly(A)+ RNA molecules did not have modified bases (caps) at their 5' termini. The relatively short poly(A) tracts, the lack of 5' cap structures, and the instability of the poly(A)+ RNA molecules isolated from M. vannielii indicate that these archaebacterial poly(A)+ RNAs more closely resemble eubacterial mRNAs than eucaryotic mRNAs.