BUNDLE SHEATH DEFECTIVE2, a novel protein required for post-translational regulation of the rbcL gene of maize

The Bundle sheath defective2 (Bsd2) gene is required for ribulose-1,5-bisph osphate carboxylase/oxygenase (Rubisco) accumulation in maize, Using a Muta tor transposable element as a molecular probe, we identified a tightly link ed restriction fragment length polymorphism that cosegregated with the bsd2 -conferred phenotype, This fragment was cloned, and sequences flanking the Mutator insertion were used to screen a maize leaf cDNA library. Using a fu ll-length cDNA clone isolated in this screen, we show that an abundant 0.6- kb transcript could be detected in wild-type plants but not in bsd2-m1 plan ts. This 0.6-kb transcript accumulated to low levels in plants carrying an allele derived from bsd2-m1 that conditions a less severe mutant phenotype. Taken together, these data strongly suggest that we have cloned the Bsd2 g ene. Sequence analysis of the full-length cDNA clone revealed a chloroplast targeting sequence and a region of homology shared between BSD2 and the Dn aJ class of molecular chaperones. This region of homology is limited to a c ysteine-rich Zn binding domain in DnaJ believed to play a role in protein-p rotein interactions. We show that BSD2 is targeted to the chloroplast but i s not involved in general photosynthetic complex assembly or protein import . In bsd2 mutants, we could not detect the Rubisco protein, but the chlorop last-encoded Rubisco large subunit transcript (rbcL) was abundant and assoc iated with polysomes in both bundle sheath and mesophyll cells. By characte rizing Bsd2 expression patterns and analyzing the bsd2-conferred phenotype, we propose a model for BSD2 in the post-translational regulation of rbcL i n maize.