Cloning and Sequencing of an Intronless Mouse S-Adenosylmethionine Decarboxylase Gene Coding for a Functional Enzyme Strongly Expressed in the Liver

Abstract

A genomic clone for a mouse S-adenosylmethionine decarboxylase (AdoMetDC) gene was isolated from a cosmid library. Surprisingly, the gene proved to be intronless. With the exception of three base substitutions (changing 2 amino acids in the deduced protein), the 1002-nucleotide sequence of the open reading frame was identical to that of mouse AdoMetDC cDNA. Moreover, the gene contained a poly(dA) tract at the 3' end and was flanked by 13-base pair direct repeats. Our findings suggest that this gene has arisen by retroposition, in which a fully processed AdoMetDC mRNA has been reverse transcribed into a DNA copy and inserted into the genome. By polymerase chain reaction, we positively identified the intronless gene in the mouse genome, and, by primer extension analysis, we proved the gene to be functional. Thus, its transcripts were found in many cell lines and tissues of the mouse and were particularly abundant in the liver. When the open reading frame of the intronless gene was expressed in Escherichia coli HT551, a strain with no AdoMetDC activity, it was found to encode a 38-kDa protein, corresponding to AdoMetDC proenzyme. Although the change of methionine 70 to isoleucine was close to the cleavage site at serine 68, this protein underwent proenzyme processing, generating a 31-kDa alpha subunit and an 8-kDa beta subunit. Importantly, the protein encoded by the intronless gene was functional, i.e. it catalyzed the decarboxylation of S-adenosylmethionine, and its specific activity was comparable with that of recombinant human AdoMetDC purified according to the same procedure.

Essential role of S‐adenosylmethionine decarboxylase in mouse embryonic development

“…It has been reported that the mouse genome contains another intronless Amd2 gene (Persson et al 1995(Persson et al , 1999. However, we have previously reported that the relative contribution of the enzyme encoded by Amd2 to total AdoMetDC activity is small, if any (Nishimura et al 1998).…”

Rapid and regulated degradation of ornithine decarboxylase

“…Thus, differences between polyamine-starved and polyamine-replete cells in the incorporation of radioactive methionine into ODC diminished progressively with a decrease in pulse-label time, reaching a minimum with a pulse time of 4 min. In similar types of studies, Persson and co-workers did not find that the difference was abolished [156] [58]. Since newly synthesized ODC is a monomer that should bind with antizyme much more easily than preformed ODC, which is mostly a dimer, it is reasonable to suggest that newly synthesized ODC turns over much more rapidly than preformed ODC.…”

A Set of Highly Conserved RNA-binding Proteins, αCP-1 and αCP-2, Implicated in mRNA Stabilization, Are Coexpressed from an Intronless Gene and Its Intron-containing Paralog

“…However, this observation could not be generalized as examples of X-linked retrotransposons originating from autosomal precursors were also observed (32). Finally, situations have been reported in which functional processed genes and their originating intron-containing genes demonstrate a similar broad range of expression (the mouse S-adenosylmethionine decarboxylase gene (Amd2) (33) and the human eukaryotic initiation factor 4E2 gene (34)). Thus, it is difficult to support general rules correlating expression patterns of originating genes and retrotransposed gene copies.…”