Ancient splice junction shadows with relation to blocks in protein structure.

Splice junction shadows (ancient exon-exon junctions) presumably reflect the existence of amino acid primary blocks which were used in the course of evolution for the construction of new proteins. The lengths of such blocks (i.e. regions between splice junctions), as the lengths of corresponding inserted or duplicated ancient exons, should be divisible by three in order to store the preexisting coding frame in the course of evolution. In this paper, we will test the hypothesis of intron-mediated recombination in a model of block molecular evolution (exon shuffling) by revealing corresponding blocks in existing database-contained coding sequences. For this purpose, we use a weight matrix prediction of ancient splice junction shadows in coding regions of the nucleotide sequences in current databases. The usage of splice junction shadows allows us to test the block evolution hypothesis in better detail in comparison with previous methods which were based only on currently existing recent exons. Our result of block length distribution at the nucleotide level shows a clear tendency to be divisible by three. At the protein level, several unexpected favorable block lengths, which are six, nine, 12 and 15 amino acids in length, were observed. Further refinements in our method for revealing splice junction shadows (structural block boundaries) might reveal peptides which probably maintain stable folds in different structures. The latter can in turn be used for protein structure prediction.