NUCLEAR BASIC-PROTEINS IN SPERMIOGENESIS

In animal species, spermiogenesis, the late stage of spermatogenesis, is characterized by a dramatic remodelling of chromatin which involves morphological changes and various modifications in the nature of the nuclear basic proteins. According to the evolution of species, three s ituations can be observed: a) persistence of somatic histones or appea rance of sperm-specific histones; b) direct replacement of histones by generally smaller and more basic proteins called protamines; and c) o ccurrence of a double nuclear basic protein transition: histones are n ot directly replaced by protamines but by intermediate basic proteins which are themselves replaced by one or several protamines. However, i n some species, two kinds of intermediate basic proteins can be distin guished in spermatid nuclei: transition proteins and protamine precurs ors. Whereas transition proteins are not structurally related either t o histones or to protamines, protamine precursors are further processe d at the end of spermiogenesis to give rise to the mature protamine. T he molecular characteristics of the protamines as well as number of pr otamine types present in the spermatozoon vary from species to species . In some cases, protamine-encoding genes, although present, are not e xpressed to a significant level. The diversity and the precise functio n of intermediate basic proteins remain open to discussion. Some of th em are the precursors of protamines but the mechanism, sequential or n ot, as well as the enzyme(s) involved in the proteolytic processing, r emain to be discovered. ((C) Societe francaise de biochimie et biologi e moleculaire/Elsevier, Paris).