Project description:Bigelowiella natans is a marine chlorarachniophyte whose plastid was acquired secondarily via endosymbiosis with a green alga. Integrating a photosynthetic endosymbiont within the host metabolism on route to plastid evolution would require the acquisition of strategies for coping with changes in light intensity and modifications of host genes to appropriately respond to changes in photosynthetic metabolism. To investigate the transcriptional response to light intensity in chlorarachniophytes, we conducted an RNA-seq experiment to identify differentially-expressed genes following four-hour shift to high or very-low light. A shift to high light altered the expression of over 2000 genes, many involved with photosynthesis, primary metabolism, and reactive-oxygen scavenging. These changes are related to an attempt to optimize photosynthesis and increase energy sinks for excess reductant, while minimizing photo-oxidative stress. A transfer to very-low light resulted in a lower photosynthetic performance and metabolic alteration, reflecting an energy-limited state. Genes located on the nucleomorph, the vestigial nucleus in the plastid, had few changes in expression in either light treatment, indicating this organelle has relinquished most transcriptional control to the nucleus. Overall, during plastid origin, both host and transferred endosymbiont genes evolved a harmonized transcriptional network to respond to a classic photosynthetic stress.
Project description:Nucleomoprhs are remnants of secondary endosymbiotic events between two eukaryote cells wherein the endosymbiont has retained its eukaryotic nucleus. Nucleomoprhs have evolved at least twice independently, in chlorarachniophytes and cryptophytes, yet they have converged on a remarkably similar genomic architecture, characterized by the most extreme compression and minituarization among all known eukaryotes. Previous computational studies have suggested that nucleomorph chromatin likely exhibits a number of divergent features. In this work, we provide the first maps of open chromatin, active transcription, and three-dimensional genome architecture in the nucleomorph of the chlorarachniophyte Bigelowiella natans
Project description:The regulation of gene expression and RNA maturation underlies fundamental processes such as cell homeostasis, development and stress acclimation. The biogenesis and modification of RNA is tightly controlled by an array of regulatory RNAs and nucleic acid-binding proteins. While the role of small RNAs (sRNAs) in gene expression has been studied in-depth in select model organisms, little is known about sRNA biology across the eukaryotic tree of life. We used deep sequencing to explore the repertoires of sRNAs encoded by the miniaturized, endosymbiotically-derived ‘nucleomorph’ genomes of two single-celled algae, the cryptophyte Guillardia theta and the chlorarachniophyte Bigelowiella natans. A total of 32.3 and 35.3 million reads were generated from G. theta and B. natans, respectively. In G. theta, we identified nucleomorph U1, U2 and U4 spliceosomal RNAs (snRNAs) as well as 11 C/D box small nucleolar RNAs (snoRNAs), five of which have potential plant and animal homologs. The snoRNAs are predicted to perform 2’-O methylation of rRNA (but not snRNA). In B. natans, we found previously undetected RNA components of the nucleomorph spliceosome (U4 snRNA) and ribosome (5S rRNA), along with six orphan sRNAs. Analysis of chlorarachniophyte snRNAs shed light on the removal of the miniature 18-21 nt introns found in B. natans nucleomorph genes. Neither of the nucleomorph genomes appears to encode RNA pseudouridylation machinery, and U5 snRNA cannot be found in the cryptophyte. Considering the central roles of U5 snRNA and RNA modifications in other organisms, cytoplasm-to-nucleomorph RNA shuttling in cryptophyte algae is a distinct possibility.