Group I Introns and Their Maturases: Uninvited, but Welcome Guests
Homing endonucleases are a class of invasive genetic elements that use several elegant solutions to ensure their survival in natural populations. Like all successful mobile entities, homing endonucleases must either reduce the deleterious effects of insertion within essential genes of host genomes or be lost. Many homing endonuclease genes have solved this problem by colonizing self-splicing group I introns. This association makes homing endonuclease genes phenotypically “silent” since they are spliced out and thus absent from the mature mRNA of the invaded gene. Through this union, homing endonucleases and introns have co-evolved into a “hybrid” mobile element providing the introns with a mechanism to propagate themselves in a population. Remarkably, in some cases within fungal mitochondrial genomes, homing endonucleases have adapted to facilitate splicing of their encoding introns and contribute to the host’s regulation of the invaded gene. This novel adaptation, termed maturase activity, has likely served to ensure their fixture in mitochondrial and, perhaps, other genomes. In this chapter, we will review what is known concerning the mechanism of group I intron-encoded protein-assisted splicing. In addition, we will summarize new studies of both mobility and maturase functions that have resulted in a better understanding of how a single polypeptide carries out diverse and unrelated activities. Principles derived from maturase systems are likely to apply to numerous other multi-functional proteins that participate in diverse metabolic pathways.
KeywordsMitochondrial Genome Genetic Element Genetic Engineer Mobile Element Essential Gene
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