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Date of Graduation
Master of Science (MS)
Department of Biology
Beta-amylases play an essential role in night time starch degradation in leaves. There are nine known beta-amylases (BAMs) in Arabidopsis thaliana. This research is focused on BAM9, which is localized in the chloroplasts but is unique compared to active BAM proteins. First, alignments of BAM9 orthologs and catalytically active paralogs suggest that BAM9 may not be catalytically active and may not bind to starch, but it may bind to another molecule at a remote site. Secondly, microarray data shows a peak of BAM9 expression at the dark/light transition which is contradictory to what one would expect for an active BAM protein. We propose that BAM9 is not directly involved in starch degradation. Starch quantification results show a starch accumulation phenotype in a bam9 knockout mutant compared to the wild type (WT), implying that BAM9 is functional in starch metabolism. Amylase assays using pure BAM9 protein expressed in E. coli confirmed that BAM9 is catalytically inactive with starch, maltotriose, maltopentose, and maltohexose as the substrates. However, assays of crude leaf extracts from bam9 knockout mutants reveal greater amylase activity than WT, suggesting that BAM9 may play a regulatory role at the transcriptional or the post-transcriptional level. The transcriptional regulation hypothesis was rejected after observing that the expression of BAM3 in the bam9 mutant background was not different than WT. BAM1 and -3 are known to be phosphorylated and there is a chloroplast kinase that causes a starch accumulation phenotype when knocked out. We hypothesize that BAM9 acts upstream of this kinase, which may inactivate both BAM3 during the day and BAM1 during the night.
Steidle, Elizabeth Ann, "Investigation of the role of BAM9 in starch metabolism in Arabidopsis thaliana" (2010). Masters Theses, 2010-2019. 384.