Characterization of monocot and dicot plant S-adenosyl-L-methionine decarboxylase gene families including identification in the mRNA of a highly conserved pair of upstream overlapping open reading frames

S-Adenosyl-L-methionine decarboxylase (AdoMetDC; EC 4.1.1.50) is one of the key regulatory enzymes in the biosynthesis of polyamines, Isolation of gen omic and cDNA sequences from rice and Arabidopsis had indicated that this e nzyme is encoded by a small multigene family in monocot and dicot plants. A nalysis of rice, maize and Arabidopsis AdoMetDC cDNA species revealed that the monocot enzyme possesses an extended C-terminus relative to dicot and h uman enzymes. Interestingly, we discovered that all expressed plant AdoMetD C mRN4 5' leader sequences contain a highly conserved pair of overlapping u pstream open reading frames (uORFs) that overlap by one base, The 5' tiny u ORF consists of two or three codons and the 3' small uORF encodes 50-54 res idues. Sequences of the small uORFs are highly conserved between monocot, d icot and gymnosperm AdoMetDC mRNA species and the C-terminus of the plant s mall uORFs is conserved with the C-terminus of nematode AdoMetDC uORFs; suc h a conserved arrangement is strongly suggestive of a translational regulat ory mechanism. No introns were found in the main AdoMetDC proenzyme ORF fro m any of the plant genes encoding AdoMetDC, whereas introns were found in c onserved positions flanking the overlapping uORFs, The absence of the furth est 3' intron from the Arabidopsis gene encoding AdoMetDC? suggests that th is intron was lost recently. Reverse-transcriptase-mediated PCR analysis of the two Arabidopsis genes for AdoMetDC indicated that AdoMetDC1 is abundan t and ubiquitous, whereas the gene for AdoMetDC2 is expressed preferentiall y in leaves and inflorescences. Investigation of recently released Arabidop sis genome sequences has revealed that in addition to the two genes encodin g AdoMetDC isolated as part of the present work, four additional genes are present in Arabidopsis but they are probably not expressed.