摘要：

A. ADOUTTE 0 0 Laboratoire de Ge'ne'tique, Universite Paris XI, Centre d'Orsay , 91405 , France R. PERASSO Laboratoire de Biologie Cellulaire IV Mitochondrial mutations conferring erythromycin resistance (EE) are available in Paramecium and it is possible to obtain (by conjugation and cytoplasmic exchange) exconjugant cells containing a majority of wild-type erythromycin-sensitive (Es) mitochondria and a minority of E11 ones. In the presence of erythromycin, such 'mixed' cells progressively become resistant. This process of acquisition of resistance has been studied cytologically (on thin sections of single cells) and genetically (by evaluating, on the basis of previous data, the proportion of E " / E s mitochondrial genomes at various times). While at early stages of the process of transformation the whole mitochondrial population appears rather homogeneous, at later stages, (i.e. when the cell has resumed growth in the antibiotic-containing medium) one finds, side by side, both resistant-looking mitochondria (structurally normal) and sensitive-looking ones, showing the typical alterations induced in E s cells by erythromycin. Conversely, a progressive decrease in the number of Es genomes can be demonstrated. ERYTHROMYCIN-RESISTANT MITOCHONDRIA BY ERYTHROMYCIN IN PARAMECIUM INTRODUCTION Mutations conferring resistance to erythromycin have been isolated in Paramecium aurelia and shown to be located in mitochondrial DNA (Beale, 1969; Adoutte & Beisson, 1970; Beale, Knowles & Tait, 1972). In the course of the genetical analysis of these mutations the following observations have been made. After conjugation between an erythromycin-sensitive (Es) and an erythromycin-resistant cell (ER), if cytoplasmic exchange has occurred, the sensitive ex-conjugant harbours a majority of Es mitochondria and a minority of ER ones. When such a cell is placed in erythromycin-containing medium (ERY) it first behaves like a sensitive cell: its multiplication is blocked; but after a lag of 2-4 days, it becomes progressively resistant and resumes growth. This lag has been interpreted as reflecting the selective multiplication of the ER mitochondria until their number has become sufficient to allow the cells to resume growth (Adoutte & Beisson, 1970). A detailed genetical and cytological analysis of this process of transformation from erythromycin-sensitivity to erythromycin-resistance was carried out on the basis of the two following facts, previously established: (1) Erythromycin exerts profound effects on the structure of mitochondria in E s strains of Paramecium (disappearance of cristae, appearance of rigid plates, etc.) whereas the mitochondria of ER strains remain little or not at all affected (Adoutte, Balmefrezol, Beisson & Andre, 1972). It could thus be hoped that a distinction would be possible between ER and Es mitochondria within a mixed cell placed in erythromycin. (2) When a mixed cell (containing ER and E s mitochondria) is grown in the absence of antibiotic, Es mitochondria show a selective advantage over ER ones. Even if the initial number of Es mitochondria is low in the original cell, after a number of cell generations, no ER mitochondria can be detected: all the cells in the clone become pure sensitives. This evolution towards sensitivity follows reproducible kinetics which are a function of the relative proportion of the 2 types of mitochondria in the original mixed cell and of the genotype of the mitochondria confronted. We have established, for instance that after 20-25 fissions in non-selective medium, a cell containing 50% ER mitochondria and 50 % Es mitochondria produces progeny in which no ER mitochondria can be detected, whereas 35-40 generations are needed in E]^,2 + Es combinations (Adoutte & Beisson, 1972). We thus have a means of roughly evaluating the proportion of resistant mitochondria in a mixed cell, at any chosen time, by placing the cell in nonselective medium and determining the stage at which the clone obtained gives rise to pure sensitive cells. This paper describes in parallel the genetical and cytologic

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