REGULATION 145 



ber of chloroplasts per cell can thus be considerably reduced, and some 

 cells may eventually lose their last plast, thus giving rise to a clone of per- 

 manently white cells. Ultraviolet irradiation also causes the appearance of 

 many white cells. A mass transformation of green Euglena into white cells 

 can even be brought about by growth in the presence of streptomycin 

 (Provasoli et ah, 1951). Loss of the photosynthesis apparatus under these 

 conditions is irreversible as far as one can judge at present. However, this 

 permanent loss is preceded by a reversible lesion. When Euglena are kept 

 in streptomycin for 2 hr (1/10 of the mean generation time) and then trans- 

 ferred to normal medium, many of the descendants of the treated cells are 

 white and at each generation some green cells give rise to new white cells 

 (De Deken-Grenson and Messin, 1958). 



Certain of the white cells are apparently changed irreversibly, for their 

 progeny remains white for ever. Others, however, can produce a normal 

 green progeny after more than ten white generations. A certain unstable 

 state can thus result from a short streptomycin treatment, just as yeasts 

 which have been kept in low concentrations of acridine remain during a 

 few generations in an unstable state in which respiratory deficient cells can 

 produce either respiratory deficient or normal buds (Ephrussi and Hot- 

 tinguer, 1951). 



The hereditary loss of chloroplasts in Euglena has indeed much in com- 

 mon with the hereditary loss of the respiratory system in yeast (De Deken- 

 Grenson, 1960), Both appear as a mutation, the frequency of which can be 

 increased enormously and in a specific way by the action of chemicals, or 

 by unfavourable conditions. 



No genetical studies comparable to those made on yeast or Neurospora 

 could be applied to Euglena, because of the absence of sexual reproduction. 

 But the importance of an extrachromosomal factor is shown by the fact 

 that the mechanical loss of the last chloroplast results in permanently white 

 cells; this indicates that the factor is the chloroplast itself or one of its 

 constituents. On the other hand, it is almost certain that the formation of 

 at least certain of the chloroplastic enzymes is undernuclear genetic control, 

 since several typical mutants blocked at various stages of chlorophyll 

 synthesis have been isolated from Chlorella (Granick, 1950; Bogorad and 

 Granick, 1953). 



Although the lack of direct genetic analysis makes any conclusion un- 

 certain, it is most probable that the formation of the photosynthesis 

 apparatus, like that of the respiratory system of yeast, depends on both 

 nuclear genes and extrachromosomal factors (chloroplastic in the green 

 plants). 



What are the respective functions of the mendelian gene and of the 

 extrachromosomal factor in the formation of the respiratory enzymes in 

 yeast and mould or of chloroplastic proteins in Euglena} 



