Evidence of sequences resembling avian retrovirus long terminal repeats flanking the trout protamine gene.

Additional TATA boxes are present in the flanking regions of trout protamine genes. Their activity as promoters was assayed using an in vitro transcription system. These additional TATA boxes, together with polyadenylation signals that include the consensus AATAAA and CACTG sequences very close to the promoters, suggest that these sequences may be closely related to retroviral long terminal repeat (LTR) sequences. Other features of retroviral LTRs that are also present are short inverted repeats. The LTR-like sequences flanking the trout protamine gene show significant homology to the avian sarcoma virus LTR over a 40-bp region. The trout protamine gene falls into the relatively rare intronless class of eukaryotic genes. This suggests that the gene could have been derived from a processed gene introduced into the genome by reverse transcription of a mature mRNA. The protamine-mRNA-coding region is flanked by AACA... TGTT sequences, which might represent vestigial traces of past recombination events and whose presence supports the notion that the protamine gene sequence was of foreign origin. Recent attempts in this laboratory to transfer the protamine gene into mouse cells have resulted in a high frequency of deletions similar to those observed with constructs in which a retrovirus was used as a vector to transfect foreign DNA with promoters. The distribution of protamine genes in the animal kingdom is very sporadic, which suggests that protamine genes appeared relatively late in evolution. The nonuniform occurrence of the gene among lower vertebrates may have been the result of its horizontal transmission only to certain species, possibly by infection with retroviruses that acquired it from a different species.