Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York 14214, USA
The soybean Gsal gene encoding the heme and chlorophyll synthesis enzyme glutamate 1-semialdehyde aminotransferase (GSAT) is strongly expressed in symbiotic root nodules. The Gsal promoter contains a perfect dinucleotide repeat GAGA element that binds a nuclear factor to positively affect transcription. Using a yeast one-hybrid system, we isolated soybean nodule cDNA that encodes a protein that binds to the Gsal GAGA element. A peptide fusion comprising the yeast GAL4 activation domain and the GAGA binding protein (GBP) strongly activated a reporter HIS3 or lacZ gene under the control of the GAGA element. Furthermore, gel mobility shift assays show that GBP binds the GAGA element in vitro. GBP is a basic protein and contains a nuclear localization signal. Interestingly, the N-terminal portion is homologous to histones HI from numerous organisms. In Drosophila, binding of GAGA factor to GAGA elements relieves transcriptional repression exerted by histone HI. We speculate that soybean GBP may compete with histone HI for the activation of the Gsal gene. Northern blot analyses show that mRNA encoding GBP was expressed highly in nodules but not in uninfected roots, and thus Gbp is a regulated gene. Furthermore, the Gbp gene is expressed in leaves of dark-grown etiolated plantlets, but not after exposure to light for 24 hours. We are investigating the hypothesis that GBP compensates for light for high levels of expression of Gsal in the dark.
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