DAG, A GENE REQUIRED FOR CHLOROPLAST DIFFERENTIATION AND PALISADE DEVELOPMENT IN ANTIRRHINUM-MAJUS

We have identified a mutation at the DAG locus of Antirrhinum majus wh ich blocks the development of chloroplasts to give white leaves with g reen revertant sectors, The green areas contain normal chloroplasts wh ereas the white areas have small plastids that resemble proplastids, T he cotyledons of dark-grown dag mutant seedlings have plastids which a lso resemble proplastids, The palisade cells in the white areas of dag mutant leaves also lack their characteristic columnar shape, The DAG locus was cloned by transposon tagging: DAG encodes a novel protein wi th a predicted M(r) of 26k, which is targeted to the plastids, Cleavag e of its predicted transit peptide gives a mature protein of M(r) 20k, Screening of databases and analysis of Southern blots gave evidence t hat DAG belongs to a protein family with homology to several proteins of unknown function from plants, Expression of DAG is required for exp ression of nuclear genes affecting the chloroplasts, such as CAB and R BCS, and also for expression of the plastidial gene RPOB encoding the plastidial RNA polymerase beta subunit, indicating that it functions v ery early in chloroplast development.