EXPRESSION OF A DOMINANT-NEGATIVE FORM OF CPSRP54 INHIBITS CHLOROPLAST BIOGENESIS IN ARABIDOPSIS

The chloroplast homolog of the 54 kDa subunit of signal recognition pa rticle is required for the in vitro targeting of chlorophyll a/b bindi ng proteins (LHCP) to the thylakoid membrane. To explore the function of cpSRP54 in vivo, plants that are mutated in cpSRP54 function were g enerated. Dominant negative forms of cpSRP54 altered in single amino a cids within the conserved guanine nucleotide binding domain were expre ssed in Arabidopsis. Transformed plants contained less than 30% of the wild-type level of cpSRP54 protein. As a consequence of the reduced c pSRP54 protein content, the first emerging leaves were yellow and cont ained immature chloroplasts. Although the chlorophyll (chi) content of the leaves was reduced by 75%, the chi a/b ratio was unaffected, indi cating a role of cpSRP54 in the biogenesis of proteins besides LHCP. M any chloroplast proteins were less abundant in the first emerging leav es, including non-pigmented proteins, thylakoid proteins known to be t argeted by alternative pathways, and soluble proteins. These observati ons indicate that the cpSRP54 mutation also has a pleiotropic effect o n chloroplast biogenesis. Whereas the level of cpSRP54 remained low as the plants aged, leaves emerging subsequently had a wild-type appeara nce, suggesting that the adult plants compensated for the reduction in cpSRP54 protein.