TRANSPOSON-DISRUPTION OF A MAIZE NUCLEAR GENE, THA1, ENCODING A CHLOROPLAST SECA HOMOLOG - IN-VIVO ROLE OF CP-SECA IN THYLAKOID PROTEIN TARGETING

A nuclear mutant of maize, tha1, which exhibited defects in the transl ocation of proteins across the thylakoid membrane, was described previ ously. A transposon insertion at the tha1 locus facilitated the clonin g of portions of the tha1 gene. Strong sequence similarity with secA g enes from bacteria, pea and spinach indicates that tha1 encodes a SecA homologue (cp-SecA). The tha1-ref allele is either null or nearly so, in that tha1 mRNA is undetectable in mutant leaves and cp-SecA accumu lation is reduced greater than or equal to 40-fold. These results, in conjunction with the mutant phenotype described previously, demonstrat e that cp-SecA functions in vivo to facilitate the translocation of OE C33, PSI-F and plastocyanin hut does not function in the translocation of OEC23 and OEC16. Our results confirm predictions for cp-SecA funct ion made from the results of in vitro experiments and establish severa l new functions for cp-SecA, including roles in the targeting of a chl oroplast-encoded protein, cytochrome f, and in protein targeting in th e etioplast, a nonphotosynthetic plastid type. Our finding that the ac cumulation of properly targeted plastocyanin and cytochrome fin tha1-r ef thylakoid membranes is reduced only a few-fold despite the near or complete absence of cp-SecA suggests that cp-SecA facilitates but is n ot essential an vivo for their translocation across the membrane.