Gene expression during mammalian spermatogenesis. III. Changes in populations of mRNA during spermiogenesis

Round spermatids and elongating spermatids were purified from a suspension of mouse testicular cells by sedimentation at unit gravity coupled with density gradient centrifugation through Percoll. Following separation, the two cell types were fractionated into polysomal and non-polysomal compartments. By comparison with round spermatids, elongating spermatids contain about one-half as much cytoplasmic RNA per cell, one sixth as much poly(A)+ RNA per cell and one-half the concentration of poly (A)+ mRNA in their cytoplasm. About two-thirds of the poly(A)+ messenger RNA (mRNA) was in the non-polysomal fraction in both cell types. Polypeptides whose synthesis was directed by cell-free translation of purified mRNA from each cell fraction were analyzed by two-dimensional gel electrophoresis. At the level of detection provided by the electrophoretic methods used, the majority of peptides from the polysomal and non-polysomal compartments for each cell type were similar. However, between the two cell types, approx. 5-10% of the polypeptides in the polysomal and non-polysomal fractions differed markedly in abundance. When the polypeptides encoded by the polysomal and non-polysomal mRNA from round spermatids were compared to the polypeptides encoded in the equivalent fractions from elongating spermatids, a significant reduction in number of polypeptides from elongating spermatids was seen. The presence of specific mRNAs in the non-polysomal fraction of round spermatids and in the polysomal fraction of elongating spermatids suggests that storage of mRNA in the cytoplasm and subsequent utilization provides a source of mRNA for proteins expressed at a time during spermiogenesis when transcription has terminated.