This research focuses on the regulation of nuclear genes that encode enzymes involved in the process of photorespiration. The current emphasis is on the gene for hydroxypyruvate reductase (HPR) and on the regulated expression of this gene in cucumber seedlings and in transgenic tobacco plants. HPR gene expression is restricted to photosynthetic tissues, stimulated by light, inhibited by high CO2, and dependent on the presence of intact chloroplasts. Using an HPR cDNA as a probe, it has been shown that regulation of gene expression by both light and CO2 involves changes in transcript (mRNA) abundance, probably reflecting changes in the rate of transcription. The PI has isolated and sequenced a cucumber genomic clone for HPR that contains the entire coding region plus 2.2 kilobases of 5' flanking region and 0.8 kilobases of 3' flanking region. It has been shown that the gene, called hpr-A, is expressed in transgenic tobacco plants in an organ- specific, light-regulated manner. Here, the PI proposes to identify cis-acting regulatory elements involved in the effects of CO2 on HPR gene expression, using appropriate deletion and/or fusion constructs of the 5' flanking region, the transcribed region, and the 3' flanking region of hpr- A. The PI also plans to assay in vitro for trans-acting factors (nuclear proteins, most likely) by using mobility shirt and DNA footprinting assays. In addition, the PI intends to characterize more fully the regulatory effect on HPR gene expression of CO2 (or of a photorespiratory metabolite, probably). As one of the few research groups currently studying the regulation of photorespiratory enzymes and the first with a sequenced gene for an enzyme of the photorespiratory glycolate pathway, the PI is in a good position to contribute significantly to our understanding of the regulation of these genes by CO2. Given the importance of photorespiration to photosynthetic efficiency and hence to crop yields, results form this research should be of interest to others with more applied agronomic interests and perhaps also to those concerned about the effects of the increasing CO2 concentration in the earth's environment.
