Abstract
Gene relocation from the residual genomes of organelles to the nuclear genome still continues, although as a scaled down evolutionary phenomenon, limited in occurrence mostly to protists (sensu lato) and land plants. During this process, the structural integrity of transferred genes is usually preserved. However, the relocation of mitochondrial genes that code for respiratory chain and ribosomal proteins is sometimes associated with their fragmentation into two complementary genes. Herein, this review compiles cases of piecewise gene transfer from the mitochondria to the nucleus, and discusses hypothesized mechanistic links between the fission and relocation of those genes.
Highlights
Intercompartmental, evolutionary gene transfer from the greatly reduced genomes of mitochondria, plastids and nucleomorphs to the nuclear genome is, according to the endosymbiotic theory of organelle origin, a continuation of horizontal relocation of genes from pre-organellar endosymbionts of evolving eukaryotic cells [1,2,3,4,5,6]
Not all sequences presumed to be necessary for functional rRNAs have been identified, for example, in the mitochondrial DNA (mtDNA) of Plasmodium, suggesting that some of them may be specified by derived genes relocated to the nucleus and posttranscriptionally imported to mitochondria
Whether and in what circumstances the gene fission and intercompartmental transfer might be mechanistically linked has been a subject of controversy, and apparently depends on the physicochemical properties of a relocated gene product, the subcellular localization of its activity, and the mechanism of gene split and relocation
Summary
Intercompartmental, evolutionary gene transfer from the greatly reduced genomes of mitochondria (mtDNA), plastids and nucleomorphs to the nuclear genome is, according to the endosymbiotic theory of organelle origin, a continuation of horizontal (or lateral) relocation of genes from pre-organellar endosymbionts of evolving eukaryotic cells [1,2,3,4,5,6]. Integration of organelle or endosymbiont DNA fragments to the nuclear genome (or sometimes transfer of the entire organelle DNA), occurs in almost all eukaryotic organisms, it usually results in the generation of nuclear pseudogenes of organellar or endosymbiont genes [2,3,5,10,11,12] Such a DNA transfer may contribute to the development of novel exonic sequences in existing nuclear genes [9,13]. Gene fragmentation, when followed by combinatorial fusions, is one of the major ways to develop new molecular structures and activities [28,29] It reduces problems associated with folding large proteins, and creates additional opportunities for regulation of the expression and function of the corresponding RNA or protein heteromultimeric complexes [30,31,32]. I review piecewise gene transfers from mitochondria to the nucleus, partial nuclear and organelle gene conversions, and discuss mechanisms of gene transfer in the context of functional gene fission
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