Molecular cloning of the maize gene crp1 reveals similarity between regulators of mitochondrial and chloroplast gene expression

The maize nuclear gene crp1 is required for the translation of the chloropl ast petA and petD mRNAs and for the processing of the petD mRNA from a poly cistronic precursor. In order to understand the biochemical role of the crp 1 gene product and the interconnections between chloroplast translation and RNA metabolism, the crp1 gene and cDNA were cloned. The predicted crp1 gen e product (CRP1) is related to nuclear genes in fungi that play an analogou s role in mitochondrial gene expression, suggesting an underlying mechanist ic similarity. Analysis of double mutants that lack both chloroplast riboso mes and crp1 function indicated that CRP1 activates a site-specific endorib onuclease independently of any role it plays in translation. Antibodies pre pared to recombinant CRP1 were used to demonstrate that CRP1 is localized t o the chloroplast stroma and that it is a component of a multisubunit compl ex. The CRP1 complex is not associated detectably with either chloroplast m embranes or chloroplast ribosomes. Models for CRP1 function and its relatio nship to other activators of organellar translation are discussed.