STUDIES ON A PROTAMINE (GALLINE) FROM FOWL SPERM

A walk though vertebrate and invertebrate protamines

An updated comparative analysis of protamines and their corresponding genes is presented, including representative organisms from each of the vertebrate classes and one invertebrate (squid, Loligo opalescens). Special emphasis is placed on the implications for sperm chromatin organization and the evolutionary significance. The review is based on some of the most recent publications in the field and builds upon previously published reviews on this topic.

Cloning of bovine P1 protamine cDNA and the evolution of vertebrate P1 protamines

A bovine P1 protamine cDNA from a bull testis cDNA library was isolated utilizing a series of oligonucleotide probes. Sequence analysis showed that the cloned cDNA insert extended 317 bp to the poly(A) tail. The 51-residue 6750-dalton protamine primary translated protein is encoded within a 156-bp segment. The protamine sequence predicted from the cDNA sequence differs from that previously reported for the amino acid sequence of bovine protamine P1 by the insertion of the tripeptide Cys-Arg-Arg from residues 39-41 in the carboxy-terminal region of the mature protein. Consistent with previous hybridization analysis, nucleotide sequence comparisons showed that trout protamine cDNA was more closely related to that of bovine than to that of mouse. However, bovine P1 protamine cDNA shared greater sequence homology with mouse P1. A common nucleotide sequence of 30 bp is conserved among all three of these species. Primer extension analysis revealed that, as with trout protamine mRNAs, the majority of the untranslated portion of the mRNA lies 3' to the coding segment. Comparisons of their mRNA secondary structures by computer modeling indicate that the mRNAs fold back onto themselves, producing similar, extensively hydrogen-bonded, convoluted forms. These models support the view that translational regulation of protamine mRNA may be partially dependent on secondary structure. Southern analysis suggests that the bovine protamine P1 gene is not sex-linked and is present as one (or relatively few) copy within the bovine genome.

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.

Mapping of haploid expressed genes: genes for both mouse protamines are located on chromosome 16

Mouse spermatozoa contain two protamines with different amino acid sequences. By hybridizing Southern blots of a series of mouse-hamster somatic cell hybrids containing subsets of mouse chromosomes and a complete set of hamster chromosomes with 32P-labeled cDNAs for each mouse protamine, we assign the two mouse protamine genes to chromosome 16. This report presents the first evidence for chromosomal linkage of two sperm-specific, haploid regulated gene products.