Isolation and characterization of senescence-induced cDNAs encoding deoxyhypusine synthase and eucaryotic translation initiation factor 5A from tomato

Full-length cDNA clones encoding deoxyhypusine synthase (DHS) and eucaryoti c initiation factor 5A (eIF-5A) have been isolated from a cDNA expression l ibrary prepared from tomato leaves (Lycopersicon esculentum, cv. Match) exp osed to environmental stress. DHS mediates the first of two enzymatic react ions that activate eIF-5A by converting a conserved lysine to the unusual a mino acid, deoxyhypusine, Recombinant protein obtained by expressing tomato DHS cDNA in Escherichia coil proved capable of carrying out the deoxyhypus ine synthase reaction in vitro in the presence of eIF-5A Of particular inte rest is the finding that DHS mRNA and eIF-5A mRNA show a parallel increase in abundance in senescing tomato flowers, senescing tomato fruit, and envir onmentally stressed tomato leaves exhibiting programmed cell death. Western blot analyses indicated that DHS protein also increases at the onset of se nescence. It is apparent from previous studies with yeast and mammalian cel ls that hypusine-modified eIF-5A facilitates the translation of a subset of mRNAs mediating cell division. The present study provides evidence for sen escence-induced DHS and eIF-5A in tomato tissues that may facilitate the tr anslation of mRNA species required for programmed cell death.