Yeast ribosomal proteins: XI. Molecular analysis of two genes encoding YL41, an extremely small and basic ribosomal protein, from Saccharomyces cerevisiae

Two neuronal peptides encoded from a single transcript regulate mitochondrial complex III in Drosophila

Naturally produced peptides (<100 amino acids) are important regulators of physiology, development, and metabolism. Recent studies have predicted that thousands of peptides may be translated from transcripts containing small open-reading frames (smORFs). Here, we describe two peptides in Drosophila encoded by conserved smORFs, Sloth1 and Sloth2. These peptides are translated from the same bicistronic transcript and share sequence similarities, suggesting that they encode paralogs. Yet, Sloth1 and Sloth2 are not functionally redundant, and loss of either peptide causes animal lethality, reduced neuronal function, impaired mitochondrial function, and neurodegeneration. We provide evidence that Sloth1/2 are highly expressed in neurons, imported to mitochondria, and regulate mitochondrial complex III assembly. These results suggest that phenotypic analysis of smORF genes in Drosophila can provide a wealth of information on the biological functions of this poorly characterized class of genes.

Rules are made to be broken: a "simple" model organism reveals the complexity of gene regulation

Global methods for assaying translation have greatly improved our understanding of the protein-coding capacity of the genome. In particular, it is now possible to perform genome-wide and condition-specific identification of translation initiation sites through modified ribosome profiling methods that selectively capture initiating ribosomes. Here we discuss our recent study applying such an approach to meiotic and mitotic timepoints in the simple eukaryote, budding yeast, as an example of the surprising diversity of protein products-many of which are non-canonical-that can be revealed by such methods. We also highlight several key challenges in studying non-canonical protein isoforms that have precluded their prior systematic discovery. A growing body of work supports expanded use of empirical protein-coding region identification, which can help relieve some of the limitations and biases inherent to traditional genome annotation approaches. Our study also argues for the adoption of less static views of gene identity and a broader framework for considering the translational capacity of the genome.

Strategies and Challenges in Identifying Function for Thousands of sORF-Encoded Peptides in Meiosis

Recent genomic analyses have revealed pervasive translation from formerly unrecognized short open reading frames (sORFs) during yeast meiosis. Despite their short length, which has caused these regions to be systematically overlooked by traditional gene annotation approaches, meiotic sORFs share many features with classical genes, implying the potential for similar types of cellular functions. We found that sORF expression accounts for approximately 10-20% of the cellular translation capacity in yeast during meiotic differentiation and occurs within well-defined time windows, suggesting the production of relatively abundant peptides with stage-specific meiotic roles from these regions. Here, we provide arguments supporting this hypothesis and discuss sORF similarities and differences, as a group, to traditional protein coding regions, as well as challenges in defining their specific functions.

The gene-specific codon counting database: a genome-based catalog of one-, two-, three-, four- and five-codon combinations present in Saccharomyces cerevisiae genes

A codon consists of three nucleotides and functions during translation to dictate the insertion of a specific amino acid in a growing peptide or, in the case of stop codons, to specify the completion of protein synthesis. There are 64 possible single codons and there are 4096 double, 262 144 triple, 16 777 216 quadruple and 1 073 741 824 quintuple codon combinations available for use by specific genes and genomes. In order to evaluate the use of specific single, double, triple, quadruple and quintuple codon combinations in genes and gene networks, we have developed a codon counting tool and employed it to analyze 5780 Saccharomyces cerevisiae genes. We have also developed visualization approaches, including codon painting, combination and bar graphs, and have used them to identify distinct codon usage patterns in specific genes and groups of genes. Using our developed Gene-Specific Codon Counting Database, we have identified extreme codon runs in specific genes. We have also demonstrated that specific codon combinations or usage patterns are over-represented in genes whose corresponding proteins belong to ribosome or translation-associated biological processes. Our resulting database provides a mineable list of multi-codon data and can be used to identify unique sequence runs and codon usage patterns in individual and functionally linked groups of genes.

Database URL:http://www.cs.albany.edu/~tumu/GSCC.html

Decreased peptidyltransferase activity correlates with increased programmed -1 ribosomal frameshifting and viral maintenance defects in the yeast Saccharomyces cerevisiae

Increased efficiencies of programmed -1 ribosomal frameshifting in yeast cells expressing mutant forms of ribosomal protein L3 are unable to maintain the dsRNA "Killer" virus. Here we demonstrate that changes in frameshifting and virus maintenance in these mutants correlates with decreased peptidyltransferase activities. The mutants did not affect Ty1-directed programmed +1 ribosomal frameshifting or nonsense-mediated mRNA decay. Independent experiments demonstrate similar programmed -1 ribosomal frameshifting specific defects in cells lacking ribosomal protein L41, which has previously been shown to result in peptidyltransferase defects in yeast. These findings are consistent with the hypothesis that decreased peptidyltransferase activity should result in longer ribosome pause times after the accommodation step of the elongation cycle, allowing more time for ribosomal slippage at programmed -1 ribosomal frameshift signals.

A dispensable yeast ribosomal protein optimizes peptidyltransferase activity and affects translocation

Yeast ribosomal protein L41 is dispensable in the yeast. Its absence had no effect on polyphenylalanine synthesis activity, and a limited effect on growth, translational accuracy, or the resistance toward the antibiotic paromomycin. Removal of L41 did not affect the 60:40 S ratio, but it reduced the amount of 80 S, suggesting that L41 is involved in ribosomal subunit association. However, the two most important effects of L41 were on peptidyltransferase activity and translocation. Peptidyltransferase activity was measured as a second-order rate constant (k(cat)/K(s)) corresponding to the rate of peptide bond formation; this k(cat)/K(s) was lowered 3-fold to 1.15 min(-1) mm(-1) in the L41 mutant compared with 3.46 min(-1) mm(-1) in the wild type. Translocation was also affected by L41. Elongation factor 2 (EF2)-dependent (enzymatic) translocation of Ac-Phe-tRNA from the A- to P-site was more efficient in the absence of L41, because 50% translocation was achieved at only 0.004 microm EF2 compared with 0.02 microm for the wild type. Furthermore, the EF2-dependent translocation was inhibited by 50% at 2.5 microm of the translocation inhibitor cycloheximide in the L41 mutant compared with 1.2 microm in the wild type. Finally, the rate of EF2-independent (spontaneous) translocation was increased in the absence of L41.

Expression of a micro-protein

The smallest known open reading frame encodes the ribosomal protein L41, which in yeast is composed of only 24 amino acids, 17 of which are arginine or lysine. Because of the unique problems that might attend the translation of such a short open reading frame, we have investigated the properties and the translation of the mRNAs encoding L41. In Saccharomyces cerevisiae L41 is encoded by two linked genes, RPL41A and RPL41B. These genes give rise to mRNAs that have short 5' leaders of 18 and 22 nucleotides and rather long 3' leaders of 203 and 210 nucleotides not including their poly(A) tails. The mRNAs are translated exclusively on monosomes, suggesting that ribosomes do not remain attached to the mRNA after termination of translation. Calculations based on the abundance of ribosomes and of L41 mRNA indicate that the entire translation event, from initiation through termination, must occur in approximately 2 s. Termination of translation after only 25 codons does not subject the mRNAs encoding L41 to nonsense-mediated decay. Surprisingly, despite the L41 ribosomal protein being conserved from the archaea through the mammalia, S. cerevisiae can grow relatively normally after deletion of both RPL41A and RPL41B.

On the characterization of the putative S20-thx operon of Thermus thermophilus

A putative operon of the ribosomal proteins S20 and Thx has been determined in a 1.4 kb sequenced region of T. thermophilus genomic DNA. Both genes have a promoter sequence 29 nt upstream of ORF1, possess their own Shine-Dalgarno motifs (GGAG) and are separated by only 9 nucleotides, a feature characteristic of the compact Thermus thermophilus genome. This is a novel arrangement, since Thx is unique to the Thermus bacteria and in all other prokaryotes the S20 gene is monocistronic. Our results, in conjunction with the recent finding that Thx is located on the top of the head of the 30S subunit in a cavity between multiple RNA elements stabilizing them with its positive charge, corroborate the observation that thermophilic ribosomes require constituents with special features for their stabilization at high temperatures.

Identification of the 50S ribosomal proteins from the Eubacterium Thermus thermophilus

The total protein mixture from the 50S subunit (TP-50) of the eubacterium Thermus thermophilus was characterized after blotting onto PVDF membranes from two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and sequencing. The proteins were numbered according to their primary structure similarity with their counterparts from other species. One of them has been marked with an asterisk, namely L*23, because unlike the other known ribosomal proteins it shows a very low degree of homology. A highly acidic 5S rRNA binding protein, TL5, was characterized and compared with the available primary structure information. Proteins L1 and L4 migrate similarly on 2D-PAGE. Protein L4, essential for protein biosynthesis, is N-terminally blocked and shows a strikingly low homology to other L4 proteins. In addition to L4, two other proteins, namely L10 and L11, were found to be N-terminally blocked. In conclusion, 33 proteins from the large subunit were identified, including TL5. Homologs to rpL25 and rpL26 were not found.

Isolation and characterization of Nrf1p, a novel negative regulator of the Cdc42p GTPase in Schizosaccharomyces pombe

The Cdc42p GTPase and its regulators, such as the Saccharomyces cerevisiae Cdc24p guanine-nucleotide exchange factor, control signal-transduction pathways in eukaryotic cells leading to actin rearrangements. A cross-species genetic screen was initiated based on the ability of negative regulators of Cdc42p to reverse the Schizosaccharomyces pombe Cdc42p suppression of a S. cerevisiae cdc24(ts) mutant. A total of 32 S. pombe nrf (negative regulator of Cdc forty two) cDNAs were isolated that reversed the suppression. One cDNA, nrf1(+), encoded an approximately 15 kD protein with three potential transmembrane domains and 78% amino-acid identity to a S. cerevisiae gene, designated NRF1. A S. pombe Deltanrf1 mutant was viable but overexpression of nrf1(+) in S. pombe resulted in dose-dependent lethality, with cells exhibiting an ellipsoidal morphology indicative of loss of polarized cell growth along with partially delocalized cortical actin and large vacuoles. nrf1(+) also displayed synthetic overdose phenotypes with cdc42 and pak1 alleles. Green fluorescent protein (GFP)-Cdc42p and GFP-Nrf1p colocalized to intracellular membranes, including vacuolar membranes, and to sites of septum formation during cytokinesis. GFP-Nrf1p vacuolar localization depended on the S. pombe Cdc24p homolog Scd1p. Taken together, these data are consistent with Nrf1p functioning as a negative regulator of Cdc42p within the cell polarity pathway.

Analysis of a 26,756 bp segment from the left arm of yeast chromosome IV

The nucleotide sequence of a 26.7 kb DNA segment from the left arm of Saccharomyces cerevisiae chromosome IV is presented. An analysis of this segment revealed 11 open reading frames (ORFs) longer than 300 bp and one split gene. These ORFs include the genes encoding the large subunit of RNA polymerase II, the biotin apo-protein ligase, an ADP-ribosylation factor (ARF 2), the 'L35'-ribosomal protein, a rho GDP dissociation factor, and the sequence encoding the protein phosphatase 2A. Further sequence analysis revealed a short ORF encoding the ribosomal protein YL41B, an intron in a 5' untranslated region and an extended homology with another cosmid (X83276) located on the same chromosome. The potential biological relevance of these findings is discussed.

New open reading frames, one of which is similar to the nifV gene of Azotobacter vinelandii, found on a 12.5 kbp fragment of chromosome IV of Saccharomyces cerevisiae

The nucleotide sequence of a 12.5 kbp segment of the left arm of chromosome IV is described. Five open reading frames (ORFs) longer than 100 amino acids were detected, all of which are completely confined to the 12.5 kbp region. Two ORFs (D1271 and D1286) correspond to previously sequenced genes (PPH22 and VMA1 or TFP1, respectively). ORF D1298 shows the characteristics of alpha-isopropylmalate and homocitrate synthase genes and is similar to the nifV gene of Azotobacter vinelandii. Two more ORFs have no apparent homologue in the data libraries. Conversely, two smaller ORFs of 25 and 85 amino acids encoding the ribosomal protein YL41A and an ATPase inhibitor, respectively, were detected. Although a substantial part of the 12.5 kbp fragment apparently lacks protein-coding characteristics, no other elements, such as tRNA genes or transposons, were found.

Yeast ribosomal proteins: XIV. Complete nucleotide sequences of the two genes encoding Saccharomyces cerevisiae YL16

We isolated and sequenced YL16A and YL16B encoding ribosomal protein YL16 of Saccharomyces cerevisiae. The two nucleotide sequences within coding regions retain 91.1% identity, and their predicted sequences of 176 amino acids show 93.8% identity. Out of the ribosomal protein sequences from various organisms currently available, no counterpart to YL16 could be found.

Characterization by cDNA cloning of the mRNA of a highly basic human protein homologous to the yeast ribosomal protein YL41

From a cDNA library in lambda gt11 derived from poly(A)+ mRNA of human ovarian granulosa cells, a cDNA clone lambda HG12.1, containing an EcoRI insert of 470 bp, was identified. After subcloning of the insert into pUC18, the clone pHG12.1 was obtained and sequenced. The 5'-region of the insert of pHG12.1 was extended by the polymerase chain reaction (PCR) with cloned total cDNA. Assembly of the PCR fragment with the insert of pHG12.1 yielded clone pHG12. From the first open reading frame of pHG12 the amino acid sequence for a polypeptide of 25 amino acid residues (designated HG12) was derived, which was identical in 22 residues with yeast ribosomal protein YL41. It is therefore assumed that HG12 is the first mammalian homolog of yeast ribosomal protein YL41. Transcription of DNA fragments containing the coding region of pHG12 cloned into BluescriptM13, followed by cell-free translation, yielded a polypeptide with an apparent mol.wt. of 14.5 kDa, much larger than the theoretical mol.wt. (3454 Da). The discrepancy between theoretical and apparent mol.wt. was also observed for yeast ribosomal protein YL41. Southern analysis revealed that HG12 is not specified by a single copy gene. Homology for HG12 specific sequences is observed for bovine, porcine and rat species.

Yeast ribosomal proteins: XIII. Saccharomyces cerevisiae YL8A gene, interrupted with two introns, encodes a homolog of mammalian L7

We isolated and sequenced a gene, YL8A, encoding ribosomal protein YL8 of Saccharomyces cerevisiae. It is one of the two duplicated genes encoding YL8 and is located on chromosome VII while the other is on chromosome XVI. The haploid strains carrying disrupted YL8A grew more slowly than the parent strain. The open reading frame is interrupted with two introns. The predicted amino acid sequence reveals that yeast YL8 is a homolog of mammalian ribosomal protein L7, E.coli L30 and others.

The complete sequence of a 10.8kb fragment to the right of the chromosome III centromere of Saccharomyces cerevisiae

The complete nucleotide sequence of the D10H fragment (10850 bp) was determined. The D10H fragment is located on the right arm of chromosome III near the centromere and contains the SUF2 gene. Six open reading frames (ORFs) larger than 300 bp were found. One of them is the CIT2 gene encoding the cytoplasmic citrate synthase. The others are new putative genes and show no significant similarity with any known gene. In addition two tRNA genes (Asn and Pro) and a solo delta element were identified. Two ORFs were disrupted; no peculiar phenotype was observed.

Yeast ribosomal proteins: XII. YS11 of Saccharomyces cerevisiae is a homologue to E. coli S4 according to the gene analysis

We isolated and sequenced a gene, YS11A, encoding ribosomal protein YS11 of Saccharomyces cerevisiae. YS11A is one of two functional copies of the YS11 gene, located on chromosome XVI and transcribed in a lower amount than the other copy which is located on chromosome II. The disruption of YS11A has no effect on the growth of yeast. The 5'-flanking region contains a similar sequence to consensus UASrpg and the T-rich region. The open reading frame is interrupted with an intron located near the 5'-end. The predicted amino acid sequence reveals that yeast YS11 is a homologue to E. coli S4, one of the ram proteins, three chloroplast S4s and others out of the ribosomal protein sequences currently available.

Yeast ribosomal proteins: XI. Molecular analysis of two genes encoding YL41, an extremely small and basic ribosomal protein, from Saccharomyces cerevisiae

Two genes encoding ribosomal protein YL41 were cloned from Saccharomyces cerevisiae chromosomal DNA. Both genes contain an uninterrupted region of only 75 nucleotides coding for a protein of 3.3 kD. Within the coding regions the nucleotide sequences are virtually identical, whereas in both the 5'- and 3'-flanking regions the two genes differ significantly from each other. The deduced protein shows an arginine and lysine content of 68 percent, i.e., 17 out of 25 residues, and the basic residues are evenly distributed over the molecule. When compared to the ribosomal protein sequences currently available no counterpart to YL41 could be found in prokaryotes and it seems likely that YL41 is a eukaryote-specific ribosomal protein.