Maize β-amylase7 encodes two proteins using alternative transcriptional start sites: nuclear BAM7 and plastidic BAM2.

Abstract

An unusual β-amylase7 (BAM7) gene in some angiosperms, including grasses such as maize (Zea mays), appears to encode two functionally distinct proteins; a nuclear-localized transcription factor (BAM7) and a plastid-localized starch hydrolase (BAM2). In Arabidopsis (Arabidopsis thaliana), these two proteins are encoded by separate genes on different chromosomes but their physiological functions are not well established. Using the maize BAM7 gene as a model, we detected two populations of transcripts by 5'-RACE that encode the predicted proteins. The two transcripts are apparently synthesized independently using separate core promoters about 1 kb apart, the second of which is located in the first intron of the full-length gene. The N-terminus of the shorter protein, ZmBAM7-S, begins near the 3' end of the first intron of ZmBAM7-L and starts with a predicted chloroplast transit peptide. We previously showed that ZmBAM7-S is catalytically active with properties like those of AtBAM2. Here, we report that ZmBAM7-S targets green fluorescent protein to plastids. The transcript encoding the longer protein, ZmBAM7-L, encodes an additional DNA-binding domain containing a functional nuclear localization signal. This putative dual-function gene originated at least 400 Mya, prior to the emergence of ferns, and has persisted in some angiosperms that lack a separate BAM2 gene. It appears to have been duplicated and subfunctionalized in at least four lineages of land plants, resulting in two genes resembling Arabidopsis BAM2 and BAM7. Targeting of two products from a single gene to different subcellular locations is not uncommon in plants, but it is unusual when they are predicted to serve completely different functions in the two locations.