HOMOREPEATS OF AMINO-ACIDS IN PROTEINS - A DATABASE SEARCH

Occurrence of individual amino acids and their homorepeats has been co mputed from the available protein sequence database. Significant numbe rs of homorepeats, up to a maximum of hexamers, were found for most am ino acids. Preliminary results indicate that the patterns of percentag e occurrence of individual amino acids are similar to those of their h omodimers. Higher orders, however, show deviations in the predominance profile. In the case of valine and isoleucine, the frequency of occur rence of homorepeats (beyond trimers) goes down considerably, in sharp contrast to their predominance as individual entities. Tryptophan was not found beyond homotrimeric repeats and tyrosine and cysteine mere not found in tandems beyond pentamers. Interspecies distribution profi les reveal some interesting deviations. The cysteine content in E. col i proteins was about 50% lower compared to human proteins. Very few E. coli proteins have higher-order repeats and the functional importance of these higher-order repeats has been analysed. Plant proteins have very high glutamine tandems in contrast to intermediate frequency of s ingle glutamine occurrences. This suggests a preferential selection of some amino acids and their tandems in the course of evolution to suit diverse functional and structural requirements.