Genomic and Expression Analysis of Spermatid Nuclear Transition Proteins in Sequenced Drosophila Species

Spermatid nuclear transition proteins (SNTPs) in metazoans are responsible for facilitating the transformation of the histone-based nucleosome structures found in sperm precursor nuclei into the protamine-based structures found in mature sperm. In Drosophila melanogaster, the transition protein-like protein, Tpl^94D, plays this role. This work analyzed the protein sequences of transition protein-like proteins in the original 12 sequenced Drosophila species. An extensive bioinformatics approach was used to establish Tpl^94D orthologs through NCBI open reading frame (ORF) finder (PSI-BLAST) and then through a rigorous amino acid content analysis. We show that Tpl^94D orthologs are found among species closest to D. melanogaster: D. simulans, D. sechellia, D. erecta, D. yakuba, D. ananassae, and D. pseudoobscura, but absent in more distantly related species D. persmilis, D. willistoni, D. mojavensis, D. virilis, and D. grimshawi. Cyverse’s Discovery Environment was utilized in conducting RNA-Seq analysis on testis and ovary transcriptome data found for D. melanogaster, D. simulans, D. yakuba, D. ananassae, and D. pseudoobscura. Differential gene expression analysis indicated high expression for all Tpl^94D orthologs found in the testes. Further RNA-Seq analysis through NIH Isoform Two-Step Analysis (IUTA) revealed higher expression in the testes for D. melanogaster Tpl^94D isoform A over D. melanogaster Tpl^94D isoform B. Additionally, we show that the Tpl^94D orthologs and Drosophila protamine-like protein orthologs (MST35ba/MST35Bb) share the same highly conserved high mobility group (HMG) Box/DNA binding region.