“…The majority of proteins within mitochondria and chloroplasts, for example, are coded for by nuclear genes and are synthesized on cytoplasmic polyribosomes (35,193). For several such proteins it has been shown that incorporation into the organelle occurs posttranslationally and, therefore, free polypeptides must in some way pass through the limiting membranes (34,82,84,91,128,129,165).…”