Accelerated sequence divergence of conserved genomic elements in Drosophila melanogaster

Recent genomic sequencing of 10 additional Drosophila genomes provides a rich resource for comparative genomics analyses aimed at understanding the similarities and differences between species and between Drosophila and mammals. Using a phylogenetic approach, we identified 64 genomic elements that have been highly conserved over most of the Drosophila tree, but that have experienced a recent burst of evolution along the Drosophila melanogaster lineage. Compared to similarly defined elements in humans, these regions of rapid lineage-specific evolution in Drosophila differ dramatically in location, mechanism of evolution, and functional properties of associated genes. Notably, the majority reside in protein-coding regions and primarily result from rapid adaptive synonymous site evolution. In fact, adaptive evolution appears to be driving substitutions to unpreferred codons. Our analysis also highlights interesting noncoding genomic regions, such as regulatory regions in the gene gooseberry-neuro and a putative novel miRNA.     

Neutral substitutions occur at a faster rate in exons than in noncoding DNA in primate genomes

Point mutation rates in exons (synonymous sites) and noncoding (introns and intergenic) regions are generally assumed to be the same. However, comparative sequence analyses of synonymous substitutions in exons (81 genes) and that of long intergenic fragments (141.3 kbp) of human and chimpanzee genomes reveal a 30%-60% higher mutation rate in exons than in noncoding DNA. We propose a differential CpG content hypothesis to explain this fundamental, and seemingly unintuitive, pattern. We find that the increased exonic rate is the result of the relative overabundance of synonymous sites involved in CpG dinucleotides, as the evolutionary divergence in non-CpG sites is similar in noncoding DNA and synonymous sites of exons. Expectations and predictions of our hypothesis are confirmed in comparisons involving more distantly related species, including human-orangutan, human-baboon, and human-macaque. Our results suggest an underlying mechanism for higher mutation rate in GC-rich genomic regions, predict nonlinear accumulation of mutations in pseudogenes over time, and provide a possible explanation for the observed higher diversity of single nucleotide polymorphisms (SNPs) in the synonymous sites of exons compared to the noncoding regions.     

Molecular genetic analysis of the heterodimeric splicing factor U2AF: the RS domain on either the large or small Drosophila subunit is dispensable in vivo

The pre-mRNA splicing factor U2AF (U2 snRNP auxiliary factor) has an essential role in 3′ splice site selection. U2AF binds the intron pyrimidine tract between the branchpoint and the 3′ splice site and recruits U2 snRNP to the branch site at an early step in spliceosome assembly. Human U2AF is a heterodimer composed of large (hU2AF⁶⁵) and small (hU2AF³⁵) subunits. Both subunits contain a domain enriched in arginine–serine dipeptide repeats termed an RS domain. The two U2AF RS domains have been assigned essential and independent roles in spliceosome assembly in vitro—the hU2AF⁶⁵ RS domain is required to target U2 snRNP to the branch site and the hU2AF³⁵ RS domain is necessary for protein–protein interactions with constitutive and alternative splicing factors. We have investigated the functional requirements for the RS domains on the Drosophila U2AF homolog in vivo. In sharp contrast to its essential role in U2 snRNP recruitment in vitro, the RS domain on the Drosophila large subunit homolog (dU2AF⁵⁰) was completely dispensable in vivo. Prompted by this unexpected result, we analyzed the RS domain on the Drosophila small subunit homolog (dU2AF³⁸). Despite its requirement for enhancer-dependent splicing activity in vitro, the dU2AF³⁸ RS domain was also inessential in vivo. Finally, we have tested whether the Drosophila U2AF heterodimer requires any RS domain. Flies mutant for both the small and large subunits could not be rescued by dU2AF⁵⁰ΔRS and dU2AF³⁸ΔRS transgenes. Therefore, in contrast to the separate roles assigned to the U2AF RS domains in vitro, our genetic data suggest that they may have redundant functions in vivo.     

Evolutionary Conservation of Regulatory Elements in Vertebrate Hox Gene Clusters

Comparisons of DNA sequences among evolutionarily distantly related genomes permit identification of conserved functional regions in noncoding DNA. Hox genes are highly conserved in vertebrates, occur in clusters, and are uninterrupted by other genes. We aligned (PipMaker) the nucleotide sequences of the HoxA clusters of tilapia, pufferfish, striped bass, zebrafish, horn shark, human, and mouse, which are separated by approximately 500 million years of evolution. In support of our approach, several identified putative regulatory elements known to regulate the expression of Hox genes were recovered. The majority of the newly identified putative regulatory elements contain short fragments that are almost completely conserved and are identical to known binding sites for regulatory proteins (Transfac database). The regulatory intergenic regions located between the genes that are expressed most anteriorly in the embryo are longer and apparently more evolutionarily conserved than those at the other end of Hox clusters. Different presumed regulatory sequences are retained in either the Aα or Aβ duplicated Hox clusters in the fish lineages. This suggests that the conserved elements are involved in different gene regulatory networks and supports the duplication-deletion-complementation model of functional divergence of duplicated genes.     

Polycistronic pre-mRNA processing in vitro: snRNP and pre-mRNA role reversal in trans-splicing

Spliced leader (SL) trans-splicing in Caenorhabditis elegans attaches a 22-nucleotide (nt) exon onto the 5′ end of many mRNAs. A particular class of SL, SL2, splices mRNAs of downstream operon genes. Here we use an embryonic extract-based in vitro splicing system to show that SL2 specificity information is encoded within the polycistronic pre-mRNA, and that trans-splicing specificity is recapitulated in vitro. We define an RNA sequence required for SL2 trans-splicing, the U-rich (Ur) element, through mutational analysis and bioinformatics as a short stem–loop followed by a sequence motif, UAYYUU, located ∼50 nt upstream of the trans-splice site. Furthermore, this element is predicted in intercistronic regions of numerous operons of C. elegans and other species that use SL2 trans-splicing. We propose that the UAYYUU motif hybridizes with the 5′ splice site on the SL2 RNA to recruit the SL to the pre-mRNA. In this way, the UAYYUU motif in the pre-mRNA would serve an analogous function to the similar sequence in the U1 snRNA, which binds to the 5′ splice site of introns, effectively reversing the roles of snRNP and pre-mRNA in trans-splicing.     

Stem–loop 4 of U1 snRNA is essential for splicing and interacts with the U2 snRNP-specific SF3A1 protein during spliceosome assembly

The pairing of 5′ and 3′ splice sites across an intron is a critical step in spliceosome formation and its regulation. Interactions that bring the two splice sites together during spliceosome assembly must occur with a high degree of specificity and fidelity to allow expression of functional mRNAs and make particular alternative splicing choices. Here, we report a new interaction between stem–loop 4 (SL4) of the U1 snRNA, which recognizes the 5′ splice site, and a component of the U2 small nuclear ribonucleoprotein particle (snRNP) complex, which assembles across the intron at the 3′ splice site. Using a U1 snRNP complementation assay, we found that SL4 is essential for splicing in vivo. The addition of free U1-SL4 to a splicing reaction in vitro inhibits splicing and blocks complex assembly prior to formation of the prespliceosomal A complex, indicating a requirement for a SL4 contact in spliceosome assembly. To characterize the interactions of this RNA structure, we used a combination of stable isotope labeling by amino acids in cell culture (SILAC), biotin/Neutravidin affinity pull-down, and mass spectrometry. We show that U1-SL4 interacts with the SF3A1 protein of the U2 snRNP. We found that this interaction between the U1 snRNA and SF3A1 occurs within prespliceosomal complexes assembled on the pre-mRNA. Thus, SL4 of the U1 snRNA is important for splicing, and its interaction with SF3A1 mediates contact between the 5′ and 3′ splice site complexes within the assembling spliceosome.     

The Fimbria Gene Cluster of Nonencapsulated Haemophilus influenzae

The occurrence of fimbria gene clusters in nonencapsulated Haemophilus influenzae strains from chronic bronchitis patients (n = 58), patients with acute otitis media (n = 13), and healthy carriers (n = 12) was determined by DNA hybridization and PCR, based on sequences of fimbriate H. influenzae type b. Although an average of 18% of all nonencapsulated strains had a fimbria gene cluster consisting of hifA to hifE inserted in the chromosome between purE and pepN, differences in the frequency of fimbria cluster-positive strains were observed, depending on the source of isolates. The compositions of the fimbria gene clusters of seven strains from chronic bronchitis patients and one strain from an otitis media patient were analyzed in more detail. After enrichment for fimbria expression, the promoter of the gene cluster contained 10 TA repeats (n = 2), leading to optimal positioning between the −10 and −35 promoter regions. The promoter regions of five fimbria-negative strains were sequenced; four were found to have nine TA repeats, and one had only four TA repeats. The protein sequence of three ganglioside GM1-specific HifA adhesins consisted of conserved regions intermingled with regions of sequence diversity. hifA appeared to be flanked by intergenic regions that varied between strains and contained both direct and inverted DNA repeats. Since noncoding DNA between hifA and purE has not been found in H. influenzae type b, these DNA sequences are probably not essential for fimbria expression. An analysis of strains lacking the gene cluster revealed the presence of similar sequences in 13 of 15 strains from chronic bronchitis patients, 5 of 5 strains from otitis media patients, and 3 of 5 strains from healthy carriers. The lengths of these intergenic regions were the same for multiple isolates of strains obtained during persistent infections. The presence or absence and the composition of the fimbria gene cluster and other sequences between the flanking genes purE and pepN suggest that the fimbria gene cluster was originally contained on a mobile element.     

Biochemical identification of new proteins involved in splicing repression at the Drosophila P-element exonic splicing silencer

Splicing of the Drosophila P-element third intron (IVS3) is repressed in somatic tissues due to the function of an exonic splicing silencer (ESS) complex present on the 5′ exon RNA. To comprehensively characterize the mechanisms of this alternative splicing regulation, we used biochemical fractionation and affinity purification to isolate the silencer complex assembled in vitro and identify the constituent proteins by mass spectrometry. Functional assays using splicing reporter minigenes identified the proteins hrp36 and hrp38 and the cytoplasmic poly(A)-binding protein PABPC1 as novel functional components of the splicing silencer. hrp48, PSI, and PABPC1 have high-affinity RNA-binding sites on the P-element IVS3 5′ exon, whereas hrp36 and hrp38 proteins bind with low affinity to the P-element silencer RNA. RNA pull-down and immobilized protein assays showed that hrp48 protein binding to the silencer RNA can recruit hrp36 and hrp38. These studies identified additional components that function at the P-element ESS and indicated that proteins with low-affinity RNA-binding sites can be recruited in a functional manner through interactions with a protein bound to RNA at a high-affinity binding site. These studies have implications for the role of heterogeneous nuclear ribonucleoproteins (hnRNPs) in the control of alternative splicing at cis-acting regulatory sites.     

A genome-wide survey of human pseudogenes

We screened all intergenic regions in the human genome to identify pseudogenes with a combination of homology searches and a functionality test using the ratio of silent to replacement nucleotide substitutions (K_A/K_S). We identified 19,724 regions of which 95% ± 3% are estimated to evolve neutrally and thus are likely to encode pseudogenes. Half of these have no detectable truncation in their pseudocoding regions and therefore are not identifiable by methods that require the presence of truncations to prove nonfunctionality. A comparative analysis with the mouse genome showed that 70% of these pseudogenes have a retrotranspositional origin (processed), and the rest arose by segmental duplication (nonprocessed). Although the spread of both types of pseudogenes correlates with chromosome size, nonprocessed pseudogenes appear to be enriched in regions with high gene density. It is likely that the human pseudogenes identified here represent only a small fraction of the total, which probably exceeds the number of genes.     

Comparative Sequence of Human and Mouse BAC Clones from the mnd2 Region of Chromosome 2p13

The mnd2 mutation on mouse chromosome 6 produces a progressive neuromuscular disorder. To determine the gene content of the 400-kb mnd2 nonrecombinant region, we sequenced 108 kb of mouse genomic DNA and 92 kb of human genomic sequence from the corresponding region of chromosome 2p13.3. Three genes with the indicated sizes and intergenic distances were identified: D6Mm5e (81 kb)–787 bp–DOK (2 kb)–845 bp–LOR2 (6 kb). D6Mm5e is expressed in many tissues at very low abundance and the predicted 526-residue protein contains no known functional domains. DOK encodes the p62^dok rasGAP binding protein involved in signal transduction. LOR2 encodes a novel lysyl oxidase-related protein of 757 amino acid residues. We describe a simple search protocol for identification of conserved internal exons in genomic sequence. Evolutionary conservation proved to be a useful criterion for distinguishing between authentic exons and artifactual products obtained by exon amplification, RT–PCR, and 5′ RACE. Conserved noncoding sequence elements longer than 80 bp with 75% nucleotide sequence identity comprise ~1% of the genomic sequence in this region. Comparative analysis of this human and mouse genomic DNA sequence was an efficient method for gene identification and is independent of developmental stage or quantitative level of gene expression.     

Rapid evolutionary turnover underlies conserved lncRNA–genome interactions

Many long noncoding RNAs (lncRNAs) can regulate chromatin states, but the evolutionary origin and dynamics driving lncRNA–genome interactions are unclear. We adapted an integrative strategy that identifies lncRNA orthologs in different species despite limited sequence similarity, which is applicable to mammalian and insect lncRNAs. Analysis of the roX lncRNAs, which are essential for dosage compensation of the single X chromosome in Drosophila males, revealed 47 new roX orthologs in diverse Drosophilid species across ∼40 million years of evolution. Genetic rescue by roX orthologs and engineered synthetic lncRNAs showed that altering the number of focal, repetitive RNA structures determines roX ortholog function. Genomic occupancy maps of roX RNAs in four species revealed conserved targeting of X chromosome neighborhoods but rapid turnover of individual binding sites. Many new roX-binding sites evolved from DNA encoding a pre-existing RNA splicing signal, effectively linking dosage compensation to transcribed genes. Thus, dynamic change in lncRNAs and their genomic targets underlies conserved and essential lncRNA–genome interactions.     

Genome differences between Treponema pallidum subsp. pallidum strain Nichols and T. paraluiscuniculi strain Cuniculi A

The genome of Treponema paraluiscuniculi strain Cuniculi A was compared to the genome of the syphilis spirochete Treponema pallidum subsp. pallidum strain Nichols using DNA microarray hybridization, whole-genome fingerprinting, and DNA sequencing. A DNA microarray of T. pallidum subsp. pallidum Nichols containing all 1,039 predicted open reading frame PCR products was used to identify deletions and major sequence changes in the Cuniculi A genome. Using these approaches, deletions, insertions, and prominent sequence changes were found in 38 gene homologs and six intergenic regions of the Cuniculi A genome when it was compared to the genome of T. pallidum subsp. pallidum Nichols. Most of the observed differences were localized in tpr loci and the vicinity of these loci. In addition, 14 other genes were found to contain frameshift mutations resulting in major changes in protein sequences. Analysis of restriction target sites representing 0.34% of the total genome length and DNA sequencing of three PCR products (0.46% of the total genome length) amplified from Cuniculi A chromosomal regions and comparison to the Nichols genome revealed a sequence similarity of 98.6 to 99.3%. These results are consistent with a close genetic relationship among the T. pallidum strains and subspecies and a strong, but relatively divergent connection between the human and rabbit pathogens.