153 research outputs found
Emerging Genomic and Proteomic Evidence on Relationships Among the Animal, Plant and Fungal Kingdoms
Sequence-based molecular phylogenies have provided new models of early eukaryotic evolution. This includes the widely accepted hypothesis that animals are related most closely to fungi, and that the two should be grouped together as the Opisthokonta. Although most published phylogenies have supported an opisthokont relationship, a number of genes contain a tree-building signal that clusters animal and green plant sequences, to the exclusion of fungi. The alternative tree-building signal is especially intriguing in light of emerging data from genomic and proteomic studies that indicate striking and potentially synapomorphic similarities between plants and animals. This paper reviews these new lines of evidence, which have yet to be incorporated into models of broad scale eukaryotic evolution
Comparative genomics of cyclin-dependent kinases suggest co-evolution of the RNAP II C-terminal domain and CTD-directed CDKs
Background: Cyclin-dependent kinases (CDKs) are a large family of proteins that function in a variety of key regulatory pathways in eukaryotic cells, including control over the cell cycle and gene transcription. Among the most important and broadly studied of these roles is reversible phosphorylation of the C-terminal domain (CTD) of RNA polymerase II, part of a complex array of CTD/protein interactions that coordinate the RNAP II transcription cycle. The RNAP CTD is strongly conserved in some groups of eukaryotes, but highly degenerate or absent in others; the reasons for these differences in stabilizing selection on CTD structure are not clear. Given the importance of reversible phosphorylation for CTD-based transcription, the distribution and evolutionary history of CDKs may be a key to understanding differences in constraints on CTD structure; however, the origins and evolutionary relationships of CTD kinases have not been investigated thoroughly. Moreover, although the functions of most CDKs are reasonably well studied in mammals and yeasts, very little is known from most other eukaryotes. Results: Here we identify 123 CDK family members from animals, plants, yeasts, and four protists from which genome sequences have been completed, and 10 additional CDKs from incomplete genome sequences of organisms with known CTD sequences. Comparative genomic and phylogenetic analyses suggest that cell-cycle CDKs are present in all organisms sampled in this study. In contrast, no clear orthologs of transcription-related CDKs are identified in the most putatively ancestral eukaryotes, Trypanosoma or Giardia. Kinases involved in CTD phosphorylation, CDK7, CDK8 and CDK9, all are recovered as well-supported and distinct orthologous families, but their relationships to each other and other CDKs are not well-resolved. Significantly, clear orthologs of CDK7 and CDK8 are restricted to only those organisms belonging to groups in which the RNAP II CTD is strongly conserved. Conclusions: The apparent origins of CDK7 and CDK8, or at least their conservation as clearly recognizable orthologous families, correlate with strong stabilizing selection on RNAP II CTD structure. This suggests coevolution of the CTD and these CTD-directed CDKs. This observation is consistent with the hypothesis that CDK7 and CDK8 originated at about the same time that the CTD was canalized as the staging platform RNAP II transcription. Alternatively, extensive CTD phosphorylation may occur in only a subset of eukaryotes and, when present, this interaction results in greater stabilizing selection on both CTD and CDK sequences. Overall, our results suggest that transcription-related kinases originated after cell-cycle related CDKs, and became more evolutionarily and functionally diverse as transcriptional complexity increased. Originally published BMC Genomics, Vol. 5, No. 69, Sep 200
The largest subunit of RNA polymerase II from the Glaucocystophyta: functional constraint and short-branch exclusion in deep eukaryotic phylogeny
BACKGROUND: Evolutionary analyses of the largest subunit of RNA polymerase II (RPB1) have yielded important and at times provocative results. One particularly troublesome outcome is the consistent inference of independent origins of red algae and green plants, at odds with the more widely accepted view of a monophyletic Plantae comprising all eukaryotes with primary plastids. If the hypothesis of a broader kingdom Plantae is correct, then RPB1 trees likely reflect a persistent phylogenetic artifact. To gain a better understanding of RNAP II evolution, and the presumed artifact relating to green plants and red algae, we isolated and analyzed RPB1 from representatives of Glaucocystophyta, the third eukaryotic group with primary plastids. RESULTS: Phylogenetic analyses incorporating glaucocystophytes do not recover a monophyletic Plantae; rather they result in additional conflicts with the most widely held views on eukaryotic relationships. In particular, glaucocystophytes are recovered as sister to several amoebozoans with strong support. A detailed investigation shows that this clade can be explained by what we call "short-branch exclusion," a phylogenetic artifact integrally associated with "long-branch attraction." Other systematic discrepancies observed in RPB1 trees can be explained as phylogenetic artifacts; however, these apparent artifacts also appear in regions of the tree that support widely held views of eukaryotic evolution. In fact, most of the RPB1 tree is consistent with artifacts of rate variation among sequences and co-variation due to functional constraints related to C-terminal domain based RNAP II transcription. CONCLUSION: Our results reveal how subtle and easily overlooked biases can dominate the overall results of molecular phylogenetic analyses of ancient eukaryotic relationships. Sources of potential phylogenetic artifact should be investigated routinely, not just when obvious "long-branch attraction" is encountered
Functional Unit of the RNA Polymerase II C-Terminal Domain Lies within Heptapeptide PairsÂÂ
Unlike all other RNA polymerases, the largest subunit (RPB1) of eukaryotic DNA-dependent RNA polymerase II (RNAP II) has a C-terminal domain (CTD) comprising tandemly repeated heptapeptides with the consensus sequence Y-S-P-T-S-P-S. The tandem structure, heptad consensus, and most key functions of the CTD are conserved between yeast and mammals. In fact, all metazoans, fungi, and green plants examined to date, as well as the nearest protistan relatives of these multicellular groups, contain a tandemly repeated CTD. In contrast, the RNAP II largest subunits from many other eukaryotic organisms have a highly degenerate C terminus or show no semblance of the CTD whatsoever. The reasons for intense stabilizing selection on CTD structure in certain eukaryotes, and its apparent absence in others, are unknown. Here we demonstrate, through in vivo genetic complementation, that the essential functional unit of the yeast CTD is contained within pairs of heptapeptides. Insertion of a single alanine residue between diheptads has little phenotypic effect, while increasing the distance between diheptads produces a mostly quantitative effect on yeast cell growth. We further explore structural constraints on the CTD within an evolutionary context and propose selective mechanisms that could maintain a global tandem structure across hundreds of millions of years of eukaryotic evolution. Originally published Eukaryotic Cell, Vol. 3, No. 3, June 200
Horizontal gene transfer of epigenetic machinery and evolution of parasitism in the malaria parasite Plasmodium falciparum and other apicomplexans
Background
The acquisition of complex transcriptional regulatory abilities and epigenetic machinery facilitated the transition of the ancestor of apicomplexans from a free-living organism to an obligate parasite. The ability to control sophisticated gene expression patterns enabled these ancient organisms to evolve several differentiated forms, invade multiple hosts and evade host immunity. How these abilities were acquired remains an outstanding question in protistan biology.
Results
In this work, we study SET domain bearing genes that are implicated in mediating immune evasion, invasion and cytoadhesion pathways of modern apicomplexans, including malaria parasites. We provide the first conclusive evidence of a horizontal gene transfer of a Histone H4 Lysine 20 (H4K20) modifier, Set8, from an animal host to the ancestor of apicomplexans. Set8 is known to contribute to the coordinated expression of genes involved in immune evasion in modern apicomplexans. We also show the likely transfer of a H3K36 methyltransferase (Ashr3 from plants), possibly derived from algal endosymbionts. These transfers appear to date to the transition from free-living organisms to parasitism and coincide with the proposed horizontal acquisition of cytoadhesion domains, the O-glycosyltransferase that modifies these domains, and the primary family of transcription factors found in apicomplexan parasites. Notably, phylogenetic support for these conclusions is robust and the genes clearly are dissimilar to SET sequences found in the closely related parasite Perkinsus marinus, and in ciliates, the nearest free-living organisms with complete genome sequences available.
Conclusions
Animal and plant sources of epigenetic machinery provide new insights into the evolution of parasitism in apicomplexans. Along with the horizontal transfer of cytoadhesive domains, O-linked glycosylation and key transcription factors, the acquisition of SET domain methyltransferases marks a key transitional event in the evolution to parasitism in this important protozoan lineage
Evolution and expression of core SWI/SNF genes in red algae
Red algae are the oldest identifiable multicellular eukaryotes, with a fossil record dating back more than a billion years. During that time two major rhodophyte lineages, bangiophytes and florideophytes, have evolved varied levels of morphological complexity. These two groups are distinguished, in part, by different patterns of multicellular development, with florideophytes exhibiting a far greater diversity of morphologies. Interestingly, during their long evolutionary history, there is no record of a rhodophyte achieving the kinds of cellular and tissue‐specific differentiation present in other multicellular algal lineages. To date, the genetic underpinnings of unique aspects of red algal development are largely unexplored; however, they must reflect the complements and patterns of expression of key regulatory genes. Here we report comparative evolutionary and gene expression analyses of core subunits of the SWI/SNF chromatin‐remodeling complex, which is implicated in cell differentiation and developmental regulation in more well studied multicellular groups. Our results suggest that a single, canonical SWI/SNF complex was present in the rhodophyte ancestor, with gene duplications and evolutionary diversification of SWI/SNF subunits accompanying the evolution of multicellularity in the common ancestor of bangiophytes and florideophytes. Differences in how SWI/SNF chromatin remodeling evolved subsequently, in particular gene losses and more rapid divergence of SWI3 and SNF5 in bangiophytes, could help to explain why they exhibit a more limited range of morphological complexity than their florideophyte cousins
Comparative Genomics and Evolution of Proteins Associated with RNA Polymerase II C-Terminal Domain
The C-terminal domain (CTD) of the largest subunit of RNA polymerase II provides an anchoring point for a wide variety of proteins involved in mRNA synthesis and processing. Most of what is known about CTD-protein interactions comes from animal and yeast models. The consensus sequence and repetitive structure of the CTD is conserved strongly across a wide range of organisms, implying that the same is true of many of its known functions. In some eukaryotic groups, however, the CTD has been allowed to degenerate, suggesting a comparable lack of essential protein interactions. To date, there has been no comprehensive examination of CTD-related proteins across the eukaryotic domain to determine which of its identified functions are correlated with strong stabilizing selection on CTD structure. Here we report a comparative investigation of genes encoding 50 CTD-associated proteins, identifying putative homologs from 12 completed or nearly completed eukaryotic genomes. The presence of a canonical CTD generally is correlated with the apparent presence and conservation of its known protein partners; however, no clear set of interactions emerges that is invariably linked to conservation of the CTD. General rates of evolution, phylogenetic patterns, and the conservation of modeled tertiary structure of capping enzyme guanylyltransferase (Cgt1) indicate a pattern of coevolution of components of a transcription factory organized around the CTD, presumably driven by common functional constraints. These constraints complicate efforts to determine orthologous gene relationships and can mislead phylogenetic and informatic algorithms
Comparative Gene Expression and Physiological Analyses Reveal Molecular Mechanisms in Wound-Induced Spore Formation in the Edible Seaweed Nori
Are algal genes in nonphotosynthetic protists evidence of historical plastid endosymbioses?
<p>Abstract</p> <p>Background</p> <p>How photosynthetic organelles, or plastids, were acquired by diverse eukaryotes is among the most hotly debated topics in broad scale eukaryotic evolution. The history of plastid endosymbioses commonly is interpreted under the "chromalveolate" hypothesis, which requires numerous plastid losses from certain heterotrophic groups that now are entirely aplastidic. In this context, discoveries of putatively algal genes in plastid-lacking protists have been cited as evidence of gene transfer from a photosynthetic endosymbiont that subsequently was lost completely. Here we examine this evidence, as it pertains to the chromalveolate hypothesis, through genome-level statistical analyses of similarity scores from queries with two diatoms, <it>Phaeodactylum tricornutum </it>and <it>Thalassiosira pseudonana</it>, and two aplastidic sister taxa, <it>Phytophthora ramorum </it>and <it>P. sojae</it>.</p> <p>Results</p> <p>Contingency tests of specific predictions of the chromalveolate model find no evidence for an unusual red algal contribution to <it>Phytophthora </it>genomes, nor that putative cyanobacterial sequences that are present entered these genomes through a red algal endosymbiosis. Examination of genes unrelated to plastid function provide extraordinarily significant support for both of these predictions in diatoms, the control group where a red endosymbiosis is known to have occurred, but none of that support is present in genes specifically conserved between diatoms and oomycetes. In addition, we uncovered a strong association between overall sequence similarities among taxa and relative sizes of genomic data sets in numbers of genes.</p> <p>Conclusion</p> <p>Signal from "algal" genes in oomycete genomes is inconsistent with the chromalveolate hypothesis, and better explained by alternative models of sequence and genome evolution. Combined with the numerous sources of intragenomic phylogenetic conflict characterized previously, our results underscore the potential to be mislead by <it>a posteriori </it>interpretations of variable phylogenetic signals contained in complex genome-level data. They argue strongly for explicit testing of the different <it>a priori </it>assumptions inherent in competing evolutionary hypotheses.</p
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