Through linguistic analysis, we show that the presence of an amino acid at a given position within a proteome positively influences the presence of identical amino acids at nearby positions. We call this phenomenon 'amino acid clustering'. Clustering extends well beyond the closest neighbouring sites and is particularly pronounced for cysteine and tryptophan. Cysteine clusters preferentially form CXXC structures, and they are often involved in metal coordination or disulfide bond formation. Cysteine clustering shows a clear correlation with the growth temperature of the organism. This seems to be a general property of living organisms.
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