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A simple and efficient method is described for analyzing quantitatively multiple protein sequence alignments and finding the most conserved blocks as well as the maxima of divergence with in the set of aligned sequences. It consists of calculating the mean distance and the root-mean-square distance in each column of the multiple alignment, averaging the values in a window of defined length and plotting the results as a function of the position of the window. Due attention is paid to the presence of gaps in the columns. Several examples are provided, using the sequences of several cytochromes c, serine proteases, lysozymes and globins. Two distance matrices are compared, namely the matrix derived by Gribskov and Burgess from the Dayhoff matrix, and the Risler Structural Superposition Matrix. In each case, the divergence plots effectively point to the specific residues which are known to be essential for the catalytic activity of the proteins. In addition, the regions of maximum divergence are clearly delineated. Interestingly, they are generally observed in positions immediately flanking the most conserved blocks. The method should therefore be useful for delineating the peptide segments which will be good candidates for site-directed mutagenesis and for visualizing the evolutionary constraints along homologous polypeptide chains.

Resume Nous avons analyse les sequences de DNA en prenant comme motif elementaire des segments de longueur croissante, du codon jusqu'au gene. Nous avons pu ainsi identifier une partie des contraintes qui sont a l'origine de l'utilisation de la degenerescence du code dans chaque gene. Nos resultats montrent que la strategie d'utilisation des codons n'est pas uniquement liee aux caracteristiques de l'appareil de traduction.