Transcriptional and translational regulation of gene expression in haploid spermatids

During spermiogenesis, round spermatids undergo complex morphological, biochemical, and physiological modifications resulting in the formation of mature spermatozoa. While in round spermatids histones and non-histone proteins are replaced by transition proteins, in elongating spermatids, transition proteins are removed from the condensing chromatin and are replaced by protamines, which are the principal basic nuclear proteins of mature spermatozoa. The tightly packed DNA-protamine complexes cease transcription several days before the completion of spermiogenesis. Thus, major modifications in both nuclear and cytoplasmic structures continue throughout spermiogenesis, stringent temporal and stage-specific gene expression is a prerequisite for the correct differentiation of round spermatids into mature spermatozoa. The genes for transition proteins and protamines are transcribed in round and elongating spermatids. Transcription is regulated via methylation and trans-acting factors that bind to the TATA-box, the CRE-box, or other specific DNA sequences in the promoter region. The transcripts are stored as ribonucleoprotein particles in a translationally repressed state for several days and are translated in elongating and elongated spermatids. It has been demonstrated that, in haploid spermatids, essentially every mRNA exhibits evidence of translational repression. Translational regulation involves protein repressors that bind to the poly-A tail or specific RNA sequences located in the 3'-UTR.