On the expression of protamine genes in the testis of man and other mammals

Protamines are low molecular weight, highly basic nuclear proteins involved in the condensation of sperm chromatin. cDNA clones for human protamine 1 and 2 (PRM1 and PRM2) were used for Northern blot experiments with RNA from different human tissues. Protamine transcripts, 0.6 kb and 0.9 kb in lengths for PRM1 and PRM2, respectively, were detected only in testicular RNA. The hybridization signals though did not produce sharp bands but enclosed a minor fraction of significant smaller transcripts. When polysomal RNA fractions were used in the hybridization, these shorter transcripts, 0.45 kb in length for PRM1 and 0.7 kb for PRM2, were specifically enriched. As demonstrated by in situ hybridization on human testis sections, the transcripts of both protamine genes are restricted to the central cell layer of the tubuli seminiferi corresponding to the spatial arrangement of postmeiotic cells. This result indicates that the protamine genes in the human are postmeiotically and haploid expressed. When cDNA clones of both protamines of the boar (BPrm-1 and BPrm-2) were used for hybridization experiments with the testicular RNA of those mammalian species which lack protamine 2 in their spermatozoa, the presence of transcripts for both protamines was detected. It can be assumed that mammals in general are endowed with at least two protamine genes which are both transcribed but are translationally regulated in a species-specific manner.