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Free Content Developmental and light-regulated post-translational control of 3-hydroxy-3-methylglutaryl-CoA reductase levels in potato

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Summary

In plants, the first committed step in the cytosolic pathway for biosynthesis of isoprenoids is catalysed by 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR). We have added an eight amino-acid-residue epitope tag to a potato (Solanum tuberosum L.) HMGR isoform and expressed this novel protein (HMGR–FLAG) in transgenic plants. Despite high levels of transcript accumulation in all leaf stages of transgenic plants, high levels of HMGR–FLAG protein were found only in apical meristematic tissue, suggesting post-translational regulation of potato HMGR affected by plant development. Protein immunoblots, and determination of enzymatic activity and transcript accumulation in the HMGR–FLAG transgenic and the non-transgenic parental plant lines, show that HMGR levels decrease dramatically in the dark. Again, the mechanism of this control occurs at a post-translational level. After 2.5 h in darkness, levels of HMGR–FLAG are approximately one-half of those in plants in the light; protein levels recover rapidly when dark-treated plants are returned to the light. In non-transgenic plants, hmg transcript levels are reduced in the dark, whereas dark treatments do not affect transgene hmg transcripts expressed under the control of a constitutive promoter. Furthermore, transcripts for HMGR–FLAG remain associated with polyribosomes in dark-treated tissues. Addition of inhibitors of cysteine proteases during microsomal protein extraction is required for recovery of immunoreactive HMGR–FLAG. The epitope-tagged isozyme has been used to show for the first time that a regulated decrease in plant HMGR activity correlates closely with a loss of the HMGR protein. We have used whole plants to demonstrate that developmental and light-regulated control of HMGR occurs post-translationally in vivo.
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Keywords: epitope-tagging; isoprenoid; leaf development; light response

Document Type: Research Article

Affiliations: Plant Biology Division, Samuel Roberts Noble Foundation, PO Box 2180, Ardmore, OK 73402, USA, and

Publication date: August 1, 2000

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