Toc75 is the protein translocation channel, and there are three Toc75-related sequences in Arabidopsis. Fractionation experiments revealed that two of these sequences, atTOC75-III and atTOC75-IV, encode integral proteins of the outer envelope membrane. The third family member, atTOC75-I, is a pseudogene. atTOC75-III expression is greatest in young, rapidly expanding tissue, whereas atTOC75-IV is expressed constitutively throughout development at a lower level than atTOC75-III. The main orthologue of pea Toc75, atToc75-III, is essential for viability. Homozygous toc75-IV mutant plants have no visible phenotype. However, etioplasts from both toc75-IV seedlings and atTOC75-IV overexpressing lines, do display an abnormal phenotype.;Yellow-green plastid protein import 1 (ppi1) plants are null for atToc33, a component of the plasmid protein import apparatus. Homozygous ppi1 mutant seed were treated with the chemical mutagen ethyl methanesulfonate, and screened visually for partially or full restoration of the wild-type (green) phenotype. five mutant lines were isolated which contain recessive, heritable, single-locus mutations that suppress the ppi1phenotype. these five mutations were found to occupy two different loci, namely sp1 and sp2. All five mutations suppress the ppi1 phenotype in every respect tested. atToc33 is proposed to preferentially control the import of photosynthetic precursor proteins. Characterisation experiments demonstrated that mutant sp1 ppi1seedlings contain more photosynthetic proteins than < ppi1 and that < sp1 ppi1 chloroplasts are able to import the photosynthetic precursor, preOE33, more efficiently than ppi1 chloroplasts.