CHARACTERIZATION OF A CHLOROPLAST HOMOLOG OF THE 54-KDA SUBUNIT OF THE SIGNAL RECOGNITION PARTICLE

Proteins and RNAs homologous to components of the eukaryotic signal re cognition particle (SRP) have been identified in a number of prokaryot ic organisms. Here we report the isolation of an Arabidopsis thaliana cDNA, FFC (fifty-four chloroplast homologue), that encodes a chloropla st protein (54CP) homologous to the 54-kDa subunit of the signal recog nition particle. 54CP shares 27% identity with mammalian SRP54 and 44% identity with the Escherichia coli ffh gene product suggesting a prok aryotic origin for this gene. FFC is a nuclear gene encoding a 62-kDa cytoplasmically synthesized precursor that is capable of being importe d into isolated chloroplasts and processed to a 53-kDa stromal polypep tide. Antibodies generated against a portion of 54CP recognize an endo genous 53-kDa chloroplast protein that sediments at 4 S and 70 S, the latter form may be associated with ribosomes. The FFC transcript is mo st abundant in green shoot tissue whereas etiolated buds and roots hav e transcript levels about 30 and 10%, respectively, relative to light grown green shoots. By analogy with the eukaryotic signal recognition particle which targets secretory proteins to the endoplasmic reticulum , it is speculated that 54CP may target chloroplast proteins to either the thylakoid or envelope membranes.