Induction of a ribosome-inactivating protein upon environmental stress

Transcripts of altered abundance in RNA from unstressed and 500 mm salt-shocked Mesembryanthemum crystallinum (common ice plant) were detected by reverse-transcription differential display (RT-DD). One transcript, Rip1, was of very low abundance in unstressed plants and was strongly induced by stress. RNA blot hybridizations showed strong induction and a diurnal rhythm of transcript abundance with a maximum each day around the middle of the light phase. Rip1 encodes a reading frame of 289 amino acids (molecular mass 32,652), RIP1, with homology to single-chain ribosome inactivating proteins (rRNA N-glycosidases). The deduced amino acid sequence is 31.7% identical to pokeweed antiviral protein RIP-C (overall similarity 66.5%) with highest identity in domains of documented functional importance. RT-DD also detected mRNA for pyruvate, orthophosphate dikinase (PPDK) which has already been shown to be stress-induced in the ice plant [16]. RIP1, expressed in Escherichia coli, showed rRNA N-glycosidase activity against ice plant and rabbit reticulocyte ribosomes. The induction of Rip1 coincides with the transition period during which global changes in translation lead to adaptation of the ice plant to salt stress.