Characterization of a marsupial sperm protamine gene and its transcripts from the North American opossum (Didelphis marsupialis)

Abstract

A synthetic oligonucleotide primer, designed from marsupial protamine protein-sequence data [Balhorn, R., Corzett, M., Matrimas, J. A., Cummins, J. & Faden, B. (1989) Analysis of protamines isolated from two marsupials, the ring-tailed wallaby and gray short-tailed opossum, J. Cell. Biol. 107] was used to amplify, via the polymerase chain reaction, protamine sequences from a North American opossum (Didelphis marsupialis) cDNA. Using the amplified sequences as probes, several protamine cDNA clones were isolated. The protein sequence, predicted from the cDNA sequences, consisted of 57 amino acids, contained a large number of arginine residues and exhibited the sequence ARYR at its amino terminus, which is conserved in avian and most eutherian mammal protamines. Like the true protamines of trout and chicken, the opossum protamine lacked cysteine residues, distinguishing it from placental mammalian protamine 1 (P1 or stable) protamines. Examination of the protamine gene, isolated by polymerase-chain-reaction amplification of genomic DNA, revealed the presence of an intron dividing the protamine-coding region, a common characteristic of all mammalian P1 genes. In addition, extensive sequence identity in the 5' and 3' flanking regions between mouse and opossum sequences classify the marsupial protamine as being closely related to placental mammal P1. Protamine transcripts, in both birds and mammals, are present in two size classes, differing by the length of their poly(A) tails (either short or long). Examination of opossum protamine transcripts by Northern hybridization revealed four distinct mRNA species in the total RNA fraction, two of which were enriched in the poly(A)-rich fraction. Northern-blot analysis, using an intron-specific probe, revealed the presence of intron sequences in two of the four protamine transcripts. If expressed, the corresponding protein from intron-containing transcripts would differ from spliced transcripts by length (49 versus 57 amino acids) and would contain a cysteine residue.