Biosynthesis of Mitochondrial Porin and Insertion into the Outer Mitochondrial Membrane of Neuruspora crassa

Mitochondrial porin, the major protein of the outer mitochondrial membrane is synthesized by free cytoplasmic polysomes. The apparent molecular weight of the porin synthesized in homologous or heterologous cell-free systems is the same as that of the mature porin. Transfer in vitro of mitochondrial porin from the cytosolic fraction into the outer membrane of mitochondria could be demonstrated. Before membrane insertion, mitochondrial porin is highly sensitive to added proteinase; afterwards it is strongly protected. Binding of the precursor form to mitochondria occurs at 4°C and appears to precede insertion into the membrane. Unlike transfer of many precursor proteins into or across the inner mitochondrial membrane, assembly of the porin is not dependent on an electrical potential across the inner membrane

Gene localization by chromosome fractionation: globin genes are on at least two chromosomes and three estrogen-inducible genes are on three chromosomes.

Chicken metaphase chromosomes were partially purified by rate zonal centrifugation, and DNA was prepared from each of the fractions of the sucrose gradient. The DNA was digested with various restriction enzymes and subjected to electrophoresis in agarose gels. The DNA was transferred to nitrocellulose filters (as described by Southern), and the filters were hybridized with cDNA probes. Four globin genes alpha A, alpha D, beta, and rho or epsilon are located on at least two chromosomes, and three of the estrogen-inducible genes of the hen oviduct--ovalbumin, ovomucoid, and transferrin--are on three different chromosomes. These experiments also confirm our earlier assignment of the endogenous viral sequence related to Rous-associated virus-0 to a separate (and larger) chromosome than the cellular sequence related to the transforming gene of avian sarcoma virus (cellular sarc), although it now appears that cellular sarc is on a small macrochromosome, rather than on a microchromosome