  Studies of coevolution of amino acids within and between proteins have
revealed two types of coevolving units: coevolving contacts, which are pairs of
amino acids distant along the sequence but in contact in the three-dimensional
structure, and sectors, which are larger groups of structurally connected amino
acids that underlie the biochemical properties of proteins. By reconciling two
approaches for analyzing correlations in multiple sequence alignments, we link
these two findings together and with coevolving units of intermediate size,
called `sectons', which are shown to provide additional information. By
extending the analysis to the co-occurrence of orthologous genes in bacterial
genomes, we also show that the methods and results are general and relevant
beyond protein structures.
