Learning to live together: mutualism between self-splicing introns and their hosts

Group I and II introns can be considered as molecular parasites that interrupt protein-coding and structural RNA genes in all domains of life. They function as self-splicing ribozymes and thereby limit the phenotypic costs associated with disruption of a host gene while they act as mobile DNA elements to promote their spread within and between genomes. Once considered purely selfish DNA elements, they now seem, in the light of recent work on the molecular mechanisms regulating bacterial and phage group I and II intron dynamics, to show evidence of co-evolution with their hosts. These previously underappreciated relationships serve the co-evolving entities particularly well in times of environmental stress

Identification and evolution of fungal mitochondrial tyrosyl-tRNA synthetases with group I intron splicing activity

The bifunctional Neurospora crassa mitochondrial tyrosyl-tRNA synthetase (CYT-18 protein) both aminoacylates mitochondrial tRNATyr and acts as a structure-stabilizing splicing cofactor for group I introns. Previous studies showed that CYT-18 has distinct tRNATyr and group I intron-binding sites, with the latter formed by three small “insertions” in the nucleotide-binding fold and other structural adaptations compared with nonsplicing bacterial tyrosyl-tRNA synthetases. Here, analysis of genomic sequences shows that mitochondrial tyrosyl-tRNA synthetases with structural adaptations similar to CYT-18's are uniquely characteristic of fungi belonging to the subphylum Pezizomycotina, and biochemical assays confirm group I intron splicing activity for the enzymes from several of these organisms, including Aspergillus nidulans and the human pathogens Coccidioides posadasii and Histoplasma capsulatum. By combining multiple sequence alignments with a previously determined cocrystal structure of a CYT-18/group I intron RNA complex, we identify conserved features of the Pezizomycotina enzymes related to group I intron and tRNA interactions. Our results suggest that mitochondrial tyrosyl-tRNA synthetases with group I intron splicing activity evolved during or after the divergence of the fungal subphyla Pezizomycotina and Saccharomycotina by a mechanism involving the concerted differentiation of preexisting protein loop regions. The unique group I intron splicing activity of these fungal enzymes may provide a new target for antifungal drugs

Aminoacyl-transfer RNA synthetase deficiency promotes angiogenesis via the unfolded protein response pathway

Objective—Understanding the mechanisms regulating normal and pathological angiogenesis is of great scientific and clinical interest. In this report, we show that mutations in 2 different aminoacyl-transfer RNA synthetases, threonyl tRNA synthetase (tarsy58) or isoleucyl tRNA synthetase (iarsy68), lead to similar increased branching angiogenesis in developing zebrafish. Approach and Results—The unfolded protein response pathway is activated by aminoacyl-transfer RNA synthetase deficiencies, and we show that unfolded protein response genes atf4, atf6, and xbp1, as well as the key proangiogenic ligand vascular endothelial growth factor (vegfaa), are all upregulated in tarsy58 and iarsy68 mutants. Finally, we show that the protein kinase RNA-like endoplasmic reticulum kinase–activating transcription factor 4 arm of the unfolded protein response pathway is necessary for both the elevated vegfaa levels and increased angiogenesis observed in tarsy58 mutants. Conclusions—Our results suggest that endoplasmic reticulum stress acts as a proangiogenic signal via unfolded protein response pathway–dependent upregulation of vegfaa.Daniel Castranova, Andrew E. Davis, Brigid D. Lo, Mayumi F. Miller, Paul J. Paukstelis, Matthew R. Swift, Van N. Pham, Jesús Torres-Vázquez, Kameha Bell, Kenna M. Shaw, Makoto Kamei, Brant M. Weinstei