Guanylate kinase is a critical enzyme in the biosynthesis of nucleotides. It catalyzes the reaction (d)GMP + ATP => (d)GDP + ADP. This step is very important in the recovery of cyclic-GMP, and the balance of ATP and GTP concentration within the cell. Therefore, guanylate kinase is thought to be a fundamental enzyme that is important in second-messenger signal transduction pathways. A fragment of a presumptive guanylate kinase gene (designated Cap1-1U) was isolated by differential display RT-PCR during investigations into the effects of drought and osmotic stress on gene expression in sunflower. The full-length cDNA of 1547 bp, which encodes 382 amino acids (predicted 42 kDa), was cloned by Rapid Amplification of cDNA Ends (RACE). Searching electronic databases showed that the deduced amino acid sequence shared a high degree of homology with guanylate kinases. Sunflower guanylate kinase (designated SGK) contains the six highly conserved active-site domains (i.e., three "kinase" domains and three nucleotide-binding domains) known from all other guanylate kinases studied to date. Using quantitative RT-PCR, the expression of SGK was confirmed to be up-regulated in drought-treated leaves, seedling roots and shoots, but not in these same organs when plants were exposed to high salt concentrations. This implies that the expression of SGK may be specifically related to signaling the onset and/or effect of water deficit throughout the plant. Further characterization of SGK (e.g., promoter studies and protein expression) will be very helpful in understanding the function of guanylate kinase in plant cell stress-response.

Key words: differential display, drought stress, guanylate kinase, signal transduction, sunflower