Parallels in rRNA processing: conserved features in the processing of the internal transcribed spacer 1 in the pre-rRNA from Schizosaccharomyces pombe

RNase III, Ribosome Biogenesis and Beyond

The ribosome is the universal catalyst for protein synthesis. Despite extensive studies, the diversity of structures and functions of this ribonucleoprotein is yet to be fully understood. Deciphering the biogenesis of the ribosome in a step-by-step manner revealed that this complexity is achieved through a plethora of effectors involved in the maturation and assembly of ribosomal RNAs and proteins. Conserved from bacteria to eukaryotes, double-stranded specific RNase III enzymes play a large role in the regulation of gene expression and the processing of ribosomal RNAs. In this review, we describe the canonical role of RNase III in the biogenesis of the ribosome comparing conserved and unique features from bacteria to eukaryotes. Furthermore, we report additional roles in ribosome biogenesis re-enforcing the importance of RNase III.

Molecular diversity of internal transcribed spacer among the monoconidial isolates of Magnaporthe oryzae isolated from rice in Southern Karnataka, India

Blast disease of rice plant is caused by Magnaporthe oryzae (anamorph Pyricularia oryzae). This disease is recognized to be one of the most serious diseases of rice crop around the world. A total of 72 monoconidial isolates of M. oryzae obtained from blast disease samples collected around Southern Karnataka were characterized using internal transcribed spacers of the ribosomal DNA sequences. These were analyzed by comparing with already deposited sequences in GenBank database. It helped in diagnosing the invasive pathogen in all locations. Variability of rDNA sequences was found to be highly polymorphic with 0.068962 nucleotide diversity showing 6 distinct clades. 33 haplotype groups were identified with haplotype diversity of 0.8881 and Tajima's neutrality test with a D value of -1.96827 with P < 0.05 showing the presence of variations among the sequences of pathogen isolates. The Tajima's D value of less than one indicates the presence of a high number of rare alleles. Our study indicates that the pathogen might have undergone recent selection pressure because of the exposure to a large number of cultivars resulting in the evolution of rare alleles. This shows the importance of characterizing internal transcribed spacer (ITS) to know pathogen diversity and its fitness which has potential to contribute to the field of breeding for blast disease resistance.

How well do ITS rDNA sequences differentiate species of true morels (Morchella)?

Arguably more mycophiles hunt true morels (Morchella) during their brief fruiting season each spring in the northern hemisphere than any other wild edible fungus. Concerns about overharvesting by individual collectors and commercial enterprises make it essential that science-based management practices and conservation policies are developed to ensure the sustainability of commercial harvests and to protect and preserve morel species diversity. Therefore, the primary objectives of the present study were to: (i) investigate the utility of the ITS rDNA locus for identifying Morchella species, using phylogenetic species previously inferred from multilocus DNA sequence data as a reference; and (ii) clarify insufficiently identified sequences and determine whether the named sequences in GenBank were identified correctly. To this end, we generated 553 Morchella ITS rDNA sequences and downloaded 312 additional ones generated by other researchers from GenBank using emerencia and analyzed them phylogenetically. Three major findings emerged: (i) ITS rDNA sequences were useful in identifying 48/62 (77.4%) of the known phylospecies; however, they failed to identify 12 of the 22 species within the species-rich Elata Subclade and two closely related species in the Esculenta Clade; (ii) at least 66% of the named Morchella sequences in GenBank are misidentified; and (iii) ITS rDNA sequences of up to six putatively novel Morchella species were represented in GenBank. Recognizing the need for a dedicated Web-accessible reference database to facilitate the rapid identification of known and novel species, we constructed Morchella MLST (http://www.cbs.knaw.nl/morchella/), which can be queried with ITS rDNA sequences and those of the four other genes used in our prior multilocus molecular systematic studies of this charismatic genus.

Evolutionary diversification indicated by compensatory base changes in ITS2 secondary structures in a complex fungal species, Rhizoctonia solani

The rRNA cistron (18S-ITS1-5.8S-ITS2-28S) is used widely for phylogenetic analyses. Recent studies show that compensatory base changes (CBC) in the secondary structure of ITS2 correlate with genetic incompatibility between organisms. Rhizoctonia solani consists of genetically incompatible strain groups (anastomosis groups, AG) distinguished by lack of anastomosis between hyphae of strains. Phylogenetic analysis of internal transcribed spacer (ITS) sequences shows a strong correlation with AG determination. In this study, ITS sequences were reannotated according to the flanking 5.8S and 28S regions which interact during ribogenesis. One or two CBCs were detected between the ITS2 secondary structure of AG-3 potato strains as compared to AG-3 tobacco strains, and between these two strains and all other AGs. When a binucleate Rhizoctonia species related to Ceratobasidiaceae was compared to the AGs of R. solani, which were multinucleate (3-21 nuclei per cell), 1-3 CBCs were detected. The CBCs in potato strains of AG-3 distinguish them from AG-3 tobacco strains and other AGs yielding further evidence that the potato strains of AG-3 originally described as R. solani are a species distinct from other AGs. The ITS1-5.8S-ITS2 sequences were analyzed by direct sequencing of PCR products from 497 strains of AG-3 isolated from potato. The same 10 and 4 positions in ITS1 and ITS2, respectively, contained variability in 425 strains (86%). Nine different unambiguous ITS sequences (haplotypes) could be detected in a single strain by sequencing cloned PCR products indicating that concerted evolution had not homogenized the rRNA cistrons in many AG-3 strains. Importantly, the sequence variability did not affect the secondary structure of ITS2 and CBCs in AG-3.

RAC protein induces enzymatic access to the maturing 3'-end of the 25S rRNA in Schizosaccharomyces pombe

In Schizosaccharomyces pombe, interdependency between steps in the processing of the rRNAs is mediated by a large protein complex (RAC) which interacts with the non-conserved transcribed spacers. The RAC complex exhibits no nuclease activity but dramatically alters the efficiency and specificity of Pac1 nuclease cleavage, leading to the removal of the 3' external transcribed spacers (3'ETS) in the maturation of the 3'ETS region. In this study modification exclusion and S1 nuclease were used to probe the RAC protein binding site and any subsequent structural changes in the maturing region. The results indicate that, as previously observed with the ITS1 and ITS2 regions, the upper helical region in the highly conserved extended terminal hairpin constitutes a protein binding site. In turn, this interaction induces a conformational change which affords access to nuclease at the 3'-end of the maturing 25S rRNA sequence.

RNase MRP and the RNA processing cascade in the eukaryotic ancestor

BACKGROUND

Within eukaryotes there is a complex cascade of RNA-based macromolecules that process other RNA molecules, especially mRNA, tRNA and rRNA. An example is RNase MRP processing ribosomal RNA (rRNA) in ribosome biogenesis. One hypothesis is that this complexity was present early in eukaryotic evolution; an alternative is that an initial simpler network later gained complexity by gene duplication in lineages that led to animals, fungi and plants. Recently there has been a rapid increase in support for the complexity-early theory because the vast majority of these RNA-processing reactions are found throughout eukaryotes, and thus were likely to be present in the last common ancestor of living eukaryotes, herein called the Eukaryotic Ancestor.

RESULTS

We present an overview of the RNA processing cascade in the Eukaryotic Ancestor and investigate in particular, RNase MRP which was previously thought to have evolved later in eukaryotes due to its apparent limited distribution in fungi and animals and plants. Recent publications, as well as our own genomic searches, find previously unknown RNase MRP RNAs, indicating that RNase MRP has a wide distribution in eukaryotes. Combining secondary structure and promoter region analysis of RNAs for RNase MRP, along with analysis of the target substrate (rRNA), allows us to discuss this distribution in the light of eukaryotic evolution.

CONCLUSION

We conclude that RNase MRP can now be placed in the RNA-processing cascade of the Eukaryotic Ancestor, highlighting the complexity of RNA-processing in early eukaryotes. Promoter analyses of MRP-RNA suggest that regulation of the critical processes of rRNA cleavage can vary, showing that even these key cellular processes (for which we expect high conservation) show some species-specific variability. We present our consensus MRP-RNA secondary structure as a useful model for further searches.