Tp. Brutnell et al., BUNDLE SHEATH DEFECTIVE2, a novel protein required for post-translational regulation of the rbcL gene of maize, PL CELL, 11(5), 1999, pp. 849-864
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.