Enzymatic Modification of Basic Chromosomal Proteins in Developing Trout Testis
During the differentiation of trout testis cells, there are major changes in the complement of basic chromosomal proteins bound to DNA. At a specific stage in development, new, sperm-specific, highly arginine-rich proteins-the protamines-are synthesized on small cytoplasmic polysomes, phosphorylated on their seryl residues by a cyclic AMP-dependent protein kinase and rapidly transported into the nuclei of spermatid cells. After binding to chromatin, the protamines progressively replace the histones on DNA, transforming nucleohistone into the very much more compact nucleoprotamine. During this replacement the O-phosphoryl groups of protamine are removed and, in the mature sperm, de-phospho-protamines become the sole basic proteins in complex with DNA.
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