β-Amylase–Like Proteins Function as Transcription Factors in Arabidopsis, Controlling Shoot Growth and Development    

Abstract

Plants contain b-amylase-like proteins (BAMs; enzymes usually associated with starch breakdown) present in the nucleus rather than targeted to the chloroplast. They possess BRASSINAZOLE RESISTANT1 (BZR1)-type DNA binding domainsalso found in transcription factors mediating brassinosteroid (BR) responses. The two Arabidopsis thaliana BZR1-BAM proteins (BAM7 and BAM8) bind a cis-regulatory element that both contains a G box and resembles a BR-responsive element. In protoplast transactivation assays, these BZR1-BAMs activate gene expression. Structural modeling suggests that the BAM domain's glucan binding cleft is intact, but the recombinant proteins are at least 1000 times less active than chloroplastic b-amylases. Deregulation of BZR1-BAMs (the bam7bam8 double mutant and BAM8-overexpressing plants) causes altered leaf growth and development. Of the genes upregulated in plants overexpressing BAM8 and downregulated in bam7bam8 plants, many carry the cis-regulatory element in their promoters. Many genes that respond to BRs are inversely regulated by BZR1-BAMs. We propose a role for BZR1-BAMs in controlling plant growth and development through crosstalk with BR signaling. Furthermore, we speculate that BZR1-BAMs may transmit metabolic signals by binding a ligand in their BAM domain, although diurnal changes in the concentration of maltose, a candidate ligand produced by chloroplastic b-amylases, do not influence their transcription factor function.

The Enzyme-Like Domain of Arabidopsis Nuclear β-Amylases Is Critical for DNA Sequence Recognition and Transcriptional Activation

“…We previously demonstrated that both BAM7 and BAM8 fulllength proteins can bind to the BBRE motif, although transcription factor activity could only be demonstrated for BAM8 (Reinhold et al, 2011). We reasoned that this difference might lie in the enzyme-like domains.…”

“…However, only BAM8 was shown to substantially activate gene transcription via the BBRE motif in protoplasts and transgenic plants (Reinhold et al, 2011). We mined the published genome data of 33 vascular plants, two mosses, and six algae in Phytozome v9.1 (http://www.phytozome.net/) for the presence of BZR1-BAM isoforms.…”

“…BAM4 is also targeted to the chloroplasts and was proposed to exert a regulatory function, modulating the rate of starch degradation, although the molecular mechanism by which it acts is not known. BAM7 and BAM8 are both BZR1-BAMs and differ from the remaining family members (Reinhold et al, 2011). Like BAM4, they have little or no catalytic activity, but their loss has no effect on starch levels.…”

“…The genomes of vascular plants encode many proteins annotated as BAMs. The Arabidopsis thaliana genome encodes nine BAMs, not all of which are active enzymes (Fulton et al, 2008;Reinhold et al, 2011). Starch degradation in chloroplasts is mainly dependent on the catalytically active isoforms BAM1 and BAM3.…”

“…Instead of a chloroplastic target peptide, they possess an N-terminal transcription factor domain of the BZR-family type, which targets them to the nucleus and enables them to bind DNA. The fact that these proteins contain both a DNA binding domain and an enzyme-like domain led to the suggestion that they may sense a sugar ligand (Reinhold et al, 2011).…”