STUDIES ON PHENOTYPIC EXPRESSION OF RESISTANCE TO ANTIMITOCHONDRIAL INHIBITOR

The present investigation deals with the behavior of cytoplasmic determinants responsible for resistance to chloramphenicol (CAP('R)) and antimycin (ANT('R)) in a hybrid cell system. Hybrids were formed by fusing two sublines of V79 Chinese hamster cells resistant to the two inhibitors respectively. In an initial group of studies, the phenotypic expression of CAP('R) and ANT('R) determinants was examined in hybrid cells. Plating assays, performed with graded levels of ANT and CAP singly, showed that hybrid cell populations, at the time of isolation, retained the ANT('R) and CAP('R) markers. Dose-response curves for hybrid cells were intermediate to those of the parental lines. Subsequent studies were designed to see whether segregation of ANT('R) or CAP('R) from hybrid cells occurs in the presence or absence of selection. The data show that resistance to ANT('R) and CAP('R) are essentially unchanged during serial passage of the hybrid cells in drug-free medium. No segregation of the determinants is observed. However, segregation fails to occur if selective pressure is applied. Rather, there is an enhancement of resistance of both inhibitors when hybrids are serially passaged in the presence of either ANT or CAP singly. Recombination between the determinants for ANT('R) and CAP('R) does not appear to be the basis for this coordinate and positive shift. In further studies, the analysis of segregation was extended to mitochondrial populations as focal points for responses to CAP and ANT. Electron microscopy was used to characterize the effects of ANT and CAP on parental and hybrid cells in terms of drug-induced changes in mitochondrial morphology. Although the mitochondria of parental cells were sensitive to CAP and ANT respectively, it did not prove possible in hybrid cells to detect two types of mitochondria in terms of morphological pattern of mitochondria was observed among different cells of a given colony, but within single hybrid cells, all mitochondria were uniform in their responses after test exposures to ANT or CAP. These data suggest that in this system segregation fails to occur in hybrid cells between mitochondria bearing ANT('R) and CAP('R) determinants, respectively.