MSY4, a sequence-specific RNA binding protein expressed during mouse spermatogenesis

Abstract

Translational control is an important mechanism of gene regulation, especially in gametogenesis and early embryogenesis, where there are periods of time in which there is no transcription. Previously transcribed mRNAs must be translated at the correct time and in the correct sequence for development to be successfully completed. Regulatory elements have been identified within the 5' and 3' untranslated regions (UTRs) of mRNAs, but mechanisms of translational control are poorly understood. Transcription ceases midway through mouse spermatogenesis. Translational control of pre-existing mRNAs is necessary for the completion of spermatogenesis. This requires mRNA binding proteins to play a major role in spermatogenesis.Protamine 1 (Prm1) and protamine 2 (Prm2) are among the genes that are under translational control during mouse spermatogenesis. Prm1 is transcribed in round spermatids, but its mRNA is not translated until 2--8 days later in elongated spermatids. The protamines are small, arginine-rich proteins that function in the condensation of the nucleus during the later phase of spermatogenesis. Premature translation of Prm1 causes premature nuclear condensation and sterility. Translation is controlled by cis-elements in the 3' UTR and several cis- and trans-elements have been described, but the mechanism of translational control is still not clear.My dissertation focuses on one of the cis-elements within the Prm1 3' UTR and the proteins that bind to this region. I have characterized the composition of a previously described binding activity in testis extracts and cloned the gene Msy4, which encodes one of the protein components. MSY4 is highly expressed in the cytoplasm of spermatids, where translationally repressed mRNAs are stored. I have used the yeast three-hybrid system to define the Y box recognition sequence (YRS), which is the 7 nucleotide RNA binding site for MSY4 and a related protein, MSY2, and shown the necessity of the YRS for translational repression in transgenic mice. Ectopic expression of MSY4 later in mouse spermatogenesis causes dominant sterility, demonstrating the importance of MSY4 during mouse spermatogenesis.