Many proteins have repeats or runs of single amino acids. The pathogenicity of some
repeat expansions has fueled proteomic, genomic and structural explorations of homopolymeric
runs not only in human but in a wide variety of other organisms. Other types of amino
acid repetitive structures exhibit more complex patterns than homopeptides. Irrespective of
their precise organization, repetitive sequences are defined as low complexity or simple
sequences, as one or a few residues are particularly abundant. Prokaryotes show a relatively
low frequency of simple sequences compared to eukaryotes. In the latter the percentage of
proteins containing homopolymeric runs varies greatly from one group to another. For
instance, within vertebrates, amino acid repeat frequency is much higher in mammals than in
amphibians, birds or fishes. For some repeats, this is correlated with the GC-richness of the
regions containing the corresponding genes. Homopeptides tend to occur in disordered
regions of transcription factors or developmental proteins. They can trigger the formation of
protein aggregates, particularly in ‘disease’ proteins. Simple sequences seem to evolve more
rapidly than the rest of the protein/gene and may have a functional impact. Therefore, they
are good candidates to promote rapid evolutionary changes. All these diverse facets of
homopolymeric runs are explored in this review.
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