DNA packaging in mouse spermatids : Synthesis of protamine variants and four transition proteins

A comparison of the protein compositions of mouse late-step spermatids and cauda epididymal sperm has revealed that the relative distribution of the two amino acid sequence variants of mouse protamine differ markedly in spermatids and sperm. Sonication-resistant spermatids contain the two variants in a ratio of 1:1, while the ratio of these two proteins in cauda epididymal sperm is approx. 2:1. Labeling studies in vivo have shown that this difference is due, in part, to an asynchrony in the time of synthesis of the two protamine variants. Both proteins are synthesized in late-step spermatids, but synthesis of the tyrosine variant in sperm chromatin begins approximately one day before synthesis of the more predominant histidine variant. Analyses of the time of synthesis of protamine and the four transition proteins in late-step spermatids allowed us to estimate the spermatid stage in which these proteins are deposited on DNA and relate these events to the onset of sonication resistance in maturing spermatids. These results indicate that: (1) synthesis and deposition of protamine begins coincident with the onset of sonication resistance in early step 12 spermatids; (2) protamine deposition is complete by mid-step 15; and (3) synthesis of the transition proteins occurs coincident with protamine synthesis.