Genomic and expression analysis of transition proteins in Drosophila

The current study was aimed at analyzing putative protein sequences of the transition protein-like proteins in 12 Drosophila species based on the reference sequences of transition protein-like protein (Tpl (94D) ) expressed in Drosophila melanogaster sperm nuclei. Transition proteins aid in transforming chromatin from a histone-based nucleosome structure to a protamine-based structure during spermiogenesis - the post-meiotic stage of spermatogenesis. Sequences were obtained from NCBI Ref-Seq database using NCBI ORF-Finder (PSI-BLAST). Sequence alignments and analysis of the amino acid content indicate that orthologs for Tpl (94D) are present in the melanogaster species subgroup (D. simulans, D. sechellia, D. erecta, and D. yakuba), D. ananassae, and D. pseudoobscura, but absent in D. persmilis, D. willistoni, D. mojavensis, D. virilis, and D. grimshawi. Transcriptome next generation sequence (RNA-Seq) data for testes and ovaries was used to conduct differential gene expression analysis for Tpl (94D) in D. melanogaster, D. simulans, D. yakuba, D. ananassae, and D. pseudoobscura. The identified Tpl (94D) orthologs show high expression in the testes as compared to the ovaries. Additionally, 2 isoforms of Tpl (94D) were detected in D. melanogaster with isoform A being much more highly expressed than isoform B. Functional analyses of the conserved region revealed that the same high mobility group (HMG) box/DNA binding region is conserved for both Drosophila Tpl (94D) and Drosophila protamine-like proteins (MST35Ba and MST35Bb). Based on the rigorous bioinformatic approach and the conservation of the HMG box reported in this work, we suggest that the Drosophila Tpl (94D) orthologs should be classified as their own transition protein group.