mRNA-specific translational activator of cytochrome b. The cytochrome b (COB) leader RNA may represent the target sequence for CBS1 and CBS2, tethering the COB mRNA to the inner mitochondrial membrane, where cotranslational insertion of cytochrome b into the membrane can occur. Present with 556 molecules/cell in log phase SD medium. CBS1 protein devoid of mitochondrial targeting sequences can enter mitochondria in vivo, possibly due to a bypass of the mitochondrial import system. The in vitro imported CBS1 protein is functional and soluble indicating that the N-terminal part of CBS1 confers some binding to mitochondrial membranes.