TIP LOCI - 6 CHLAMYDOMONAS NUCLEAR SUPPRESSORS THAT PERMIT THE TRANSLOCATION OF PROTEINS WITH MUTANT THYLAKOID SIGNAL SEQUENCES

Mutations within the signal sequence of cytochrome f (cyt f) in Chlamy odomonas inhibit thylakoid membrane protein translocation and render c ells nonphotosynthetic. Twenty-seven suppressors of the mutant signal sequences were selected for their ability to restore photoautotrophic growth and these describe six nuclear loci named tip1 through 6 for th ylakoid insertion protein. The tip mutations restore the translocation of cytf and are not allele specific, as they suppress a number of dif ferent cytf signal sequence mutations. Tip5 and 2 may act early in cyt f translocation, while Tip1, 3, 4, and 6 are engaged later. The tip mu tations have no phenotype in the absence of a signal sequence mutation and there is genetic interaction between tip4, and tip5 suggesting an interaction of their encoded proteins. As there is overlap in the ene rgetic, biochemical and genetic requirements for the translocation of nuclear and chloroplast-encoded thylakoid proteins, the iii, mutations likely identify components of a general thylakoid protein translocati on apparatus.