Modular structural units, exons, and function in chicken lysozyme.

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Abstract
By the application of the same algorithm for finding compact structural units encoded by exons as applied previously to hemoglobin, five units, M1-M5, were identified in chicken egg white lysozyme. They consist of residues 1-30, 31-55, 56-84, 85-108, and 109-129, respectively. I call these compact structural units "modules." As in hemoglobin, modules thus identified correspond well to exons--i.e., modules M1, M2 plus M3, M4, and M5 correspond to exons 1, 2, 3, and 4 of the lysozyme gene, respectively. Localization of the catalytic sites glutamic acid-35 and aspartic acid-52 on the module M2 suggests that this module might have worked as a functional unit in a primitive lysozyme. The good correspondence between exons and modules reinforces the idea of "proteins in pieces," which was derived from the fact of "genes in pieces." The evolutionary origin of the introns in globins and lysozyme is discussed.