Arabidopsis mutants lacking the 43-and 54-kilodalton subunits of the chloroplast signal recognition particle have distinct phenotypes

The chloroplast signal recognition particle (cpSRP) is a protein complex co nsisting of 54- and 43-kD subunits encoded by the fifty-four chloroplast, w hich encodes cpSRP54 (ffc), and chaos (cao) loci, respectively. Two new nul l alleles in the ffc locus have been identified, ffc1-1 is caused by a stop codon in exon 10, while ffc1-2 has a large DNA insertion in intron 8. ffc mutants have yellow first true leaves that subsequently became green. The r eaction center proteins D1, D2, and psaA/B, as well as seven different ligh t-harvesting chlorophyll proteins (LHCPs), were round at reduced levels in the young ffc leaves but at wild-type levels in the older leaves. The abund ance of the two types of LHCP was unaffected by the mutation, while two oth ers were increased in the absence of cpSRP54. Null mutants in the cao locus contain reduced levels of the same subset of LHCP proteins as ffc mutants, but are distinguishable in four ways: young leaves are greener, the chloro phyll a/b ratio is elevated, levels of reaction center proteins are normal, and there is no recovery in the level of LHCPs in the adult plant. The dat a suggest that cpSRP54 and cpSRP43 have some nonoverlapping roles and that alternative transport pathways can compensate for the absence of a function al cpSRP.