Phosphoribulokinase from ice plant: Transcription, transcripts and protein expression during environmental stress

The expression of PRK (phosphoribulokinase, E.C.2.7.1.19) in ice plant (Mesembryanthemum crystallinum) during development and under environmental stress was studied. cDNA clones were isolated and full-length cDNAs were characterized. Ice plant PRK is contained in a 1520 nucleotide transcript including a 126 nucleotide leader sequence, a 175 nucleotide 3'-end and a 20-30 nucleotide polyA(+)-stretch. The coding region, 397 codons, specifies a protein of Mr 44 064. The mature sequence is preceded by a transit peptide of approximately 46 amino acids. The mature portion of ice plant PRK is 86.4% identical to that of spinach and, e.g., 16.2% identical to PRK from Xanthomonas flavus. Under salt stress or cold adaptation conditions, the amount of mRNA declined by a factor of approximately three within days, followed by an increase to approximately pre-stress levels. The fluctuation in mRNA amount is not reflected on the level of transcription of the gene, suggesting post-transcriptional control, nor is PRK protein amount affected significantly over the short stress period. The recovery of transcript levels for photosynthesis-related proteins after stress appears to be a general response to environmental stresses that affect water status in ice plant. We suggest that the photosynthetic machinery in this facultative halophyte is effectively buffered from damage caused by such environmental stress.