Formation of Mitochondria

The choline concentration used in the growth medium influences the density of mitochondria produced by the chol-1 mutant of Neurospora. Isopycnic centrifugation in sucrose gradients can be used to determine the density of mitochondria, and can resolve into two populations, mitochondria derived from a mixture of cells grown at low (1 pg/ml choline chloride) and high (10 #g/ml choline chloride) choline levels. In an experiment in which cells are shifted from low to high choline growth conditions, mitochondria obtained after varying time periods show a gradual decrease in density tending toward the level typical of high choline mitochondria. Over a 90-minute period of observation, during which time there is an increase of mitochondrial protein mass of ~ 50 per cent over that initially present, the mitochondria change density as a single population. These results are consistent with the view that mitochondria grow by random accretion of new lecithin into existing mitochondrial structures, and also that the mitochondrial population increases by division. It has been concluded in an earlier publication (Luck, 1963 a, b) that "new" mitochondria are formed in growing cells by growth and division of preexisting mitochondria. The present study provides a further test of this conclusion. The test is based on the observation that the density of mitochondria produced by the chol-1 mutant of Neurospora is determined, within certain limits, by the concentration of choline in the culture medium (Luck, 1965). Mitochondria produced at low choline levels (medium concentration 1 #g/ml choline chloride) are more dense than those produced at l0 /~g/ml, and artificial mixtures of light and heavy mitochondria can be resolved into separate populations by isopycnic centrifugation in sucrose gradients. A study of the density of mitochondria produced by an exponentially growing culture after a shift from low to high choline medium could provide a means of distinguishing between possible mechanisms of mitochondrial formation. If mitochondria grow and divide in a manner which randomly distributes newly added material throughout the old, it would be expected that, during the high choline growth period, the density of the entire mitochondrial population would gradually shift from a high density to the low density typical of mitochondria from cultures growing at 10 #g/ml choline. Persistence of a distinct population at low density would not be expected. If mitochondria are formed de-novo from precursors of low molecular weight, two populations should be detectable in the above experiment. New mitochondria would appear at the characteristic low density without lag, while the high density of preexisting mitochondria would remain unchanged. If mitochondria form from non-mitochondrial 461 on A ril 2, 2017 D ow nladed fom Published March 1, 1965