Putative group I introns in the eukaryote nuclear internal transcribed spacers

Group I introns: Structure, splicing and their applications in medical mycology

Group I introns are small RNAs (250-500 nt) capable of catalyzing their own splicing from the precursor RNA. They are widely distributed across the tree of life and have intricate relationships with their host genomes. In this work, we review its basic structure, self-splicing and its mechanisms of gene mobility. As they are widely found in unicellular eukaryotes, especially fungi, we gathered information regarding their possible impact on the physiology of fungal cells and the possible application of these introns in medical mycology.

Insights into Microsporidia Evolution from Early Diverging Microsporidia

Microsporidia have drastically modified genomes and cytology resulting from their high level of adaptation to intracytoplasmic parasitism. Their origins, which had long remained enigmatic, were placed within the line of Rozella, a primitive endoparasitic chytrid. These origins became more and more refined with the discovery of various parasites morphologically similar to the primitive lines of microsporidia (Metchnikovellids and Chytridiopsids) but which possess fungal-like genomes and functional mitochondria. These various parasites turn out to be distinct missing links between a large assemblage of chytrid-like rozellids and the true microsporidians, which are actually a very evolved branch of the rozellids themselves. The question of how to consider the historically known Microsporidia and the various microsporidia-like organisms within paraphyletic rozellids is discussed.

Organellar Introns in Fungi, Algae, and Plants

Introns are ubiquitous in eukaryotic genomes and have long been considered as ‘junk RNA’ but the huge energy expenditure in their transcription, removal, and degradation indicate that they may have functional significance and can offer evolutionary advantages. In fungi, plants and algae introns make a significant contribution to the size of the organellar genomes. Organellar introns are classified as catalytic self-splicing introns that can be categorized as either Group I or Group II introns. There are some biases, with Group I introns being more frequently encountered in fungal mitochondrial genomes, whereas among plants Group II introns dominate within the mitochondrial and chloroplast genomes. Organellar introns can encode a variety of proteins, such as maturases, homing endonucleases, reverse transcriptases, and, in some cases, ribosomal proteins, along with other novel open reading frames. Although organellar introns are viewed to be ribozymes, they do interact with various intron- or nuclear genome-encoded protein factors that assist in the intron RNA to fold into competent splicing structures, or facilitate the turn-over of intron RNAs to prevent reverse splicing. Organellar introns are also known to be involved in non-canonical splicing, such as backsplicing and trans-splicing which can result in novel splicing products or, in some instances, compensate for the fragmentation of genes by recombination events. In organellar genomes, Group I and II introns may exist in nested intronic arrangements, such as introns within introns, referred to as twintrons, where splicing of the external intron may be dependent on splicing of the internal intron. These nested or complex introns, with two or three-component intron modules, are being explored as platforms for alternative splicing and their possible function as molecular switches for modulating gene expression which could be potentially applied towards heterologous gene expression. This review explores recent findings on organellar Group I and II introns, focusing on splicing and mobility mechanisms aided by associated intron/nuclear encoded proteins and their potential roles in organellar gene expression and cross talk between nuclear and organellar genomes. Potential application for these types of elements in biotechnology are also discussed.

Solving an old enigma: Morellospora saccamoebae gen. nov., sp. nov. (Rozellomycota), a Sphaerita-like parasite of free-living amoebae

The Rozellomycota form a lineage basal or sister to the Fungi, ancestor of Microsporidia. Their biodiversity is very rich but remains poorly characterized. The few known species are all parasites, whether of water molds and algae (Rozella), crustaceans (Mitosporidium), or as endonuclear parasites of amoebae (Nucleophaga, Paramicrosporidium). Since the nineteenth century, intracytoplasmic parasites of various protozoa have been described as species of the same genus Sphaerita. However, it was later thought possible to separate these parasites into at least two distinct groups, those forming flagellated zoospores, prevalent in Euglena and other flagellates, and those forming immobile spores, found mainly in free-living and endozoic amoebae. Herein, we report the recovery of a strain of the free-living amoeba species Saccamoeba lacustris, naturally infected by an intracytoplasmic parasite, which under light microscope has a morphology consistent with that of Sphaerita. Biomolecular analyses were thus performed. Our results show that the intracytoplasmic parasite of Saccamoeba belongs to the same subgroup of Mitosporidium and that it forms a new genus within Rozellomycota, Morellospora, that corresponds to the former spore-forming Sphaerita-like parasites of amoebae.