Translation of cytochrome b mRNA in yeast mitochondria requires activation by the nuclear-encoded Cbs1p. According to the current model, Cbs1p tethers cyto-chrome b mRNA to the inner mitochondrial membrane via interaction with the 5'-untranslated leader. Cbs1p is predicted to be a hydrophilic protein with two hydrophobic segments near the carboxyl-terminal end, which are both too short to span the membrane. Nevertheless Cbs1p is tightly associated with the mitochondrial membrane, as shown by its behaviour in extraction experiments with taurodeoxycholate. In an attempt to define functionally important regions of Cbs1p, we created a number of mutant alleles by random and directed mutagenesis. We report that a Cbs1p mutant protein lacking the mitochondrial presequence is still able to complement a Delta cbs1 strain, suggesting that the presequence does not contain essential mitochondrial targeting information. Mutations in a cluster of positively charged amino acids at the extreme C-terminus have no effect on Cbs1p function, but removal of this segment severely impairs Cbs1p function. Truncation of 12 or more amino acids from the C-terminus results in a completely defective protein. We further show that both short hydrophobic regions are essential for Cbs1p function, although membrane association is observed even in the absence of these regions. Copyright (C) 2000 John Wiley & Sons, Ltd.