Identification of RtGST-1, a novel germ cell-specific mRNA-like transcript predominantly expressed in early previtellogenic oocytes in rainbow trout (Oncorhynchus mykiss).

Noncoding mRNA-like transcripts play important roles in a wide range of biological events such as cell differentiation and organogenesis. We report here the identification of a novel germ cell-specific mRNA-like transcript (RtGST-1) from a rainbow trout oocyte cDNA library. The novel transcript of 1,165 bp was confirmed to be full-length based on Northern blot and 5' RACE analyses. The transcript is polyadenylated but does not contain a significant open reading frame (ORF). The presumable ORF (encodes a peptide of 71 amino acids) has poor codon usage for rainbow trout and the AUG codon is in a poor context for translation initiation. RT-PCR showed that the novel gene is specifically expressed in ovaries of various stages and immature testis, whereas no transcripts of this gene were detected in mature testis and somatic tissues. Quantitative real-time PCR analysis revealed that the mRNA level of this novel gene is extremely high in early previtellogenesis stage ovaries relative to vitellogenesis stage ovaries. In situ hybridization analysis further demonstrated that the novel transcript is localized exclusively in early previtellogenic oocytes and spermatocytes. To our knowledge, this study represents the first report of a germline-specific mRNA-like transcript in fish. (c) 2007 Wiley-Liss, Inc.