LIU, XIANAN1*, EL-SAYED HASSANEIN2, AHMED BALHEIDIN2, MOHAMED EL-DOMYATI2, SHERIF EDRIS2, and WM. VANCE BAIRD1. 1Clemson University, Department of Horticulture, Clemson, SC 29634-0375; 2Department of Genetics, Faculty of Agriculture, Ain Shams University, Cairo, Egypt. - Cloning and Characterization of Guanylate Kinase from Sunflower (Helianthus annuus L.).
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