Events surrounding the early development of Euglena chloroplasts : Photoregulation of the transcription of chloroplastic and cytoplasmic ribosomal RNAs

Abstract

During light-induced chloroplast development in dark-grown resting cells of wild-type Euglena gracilis Klebs var. bacillaris Pringsheim, plastid ribosomal RNA (pl-rRNA) increases from about 2 to 25% of the total cellular RNA; concomitantly, 32PO 4 3− is incorporated linearly from 0 to 48 h. The net amount of cytoplasmic ribosomal RNA (cytrRNA) does not change during light-induced chloroplast development in wild type, but 32PO 4 3− is incorporated with a peak rate at 24–36 h, indicating that turnover of cyt-rRNA takes place during light-induced plastid development. The extent of this turnover is about 60% by 36 h of development. Such light-induced turnover of cyt-rRNA also occurs in resting cells of dark-grown W 3BUL, a mutant of Euglena which lacks detectable plastid DNA and protochlorophyll(ide), with similar kinetics but to one-third the extent of wild-type cells. Labeling of cyt-rRNA at 3 h of development is primarily in the polysome fraction compared with the monosomes, indicating that newly synthesized ribosomes are immediately active in protein synthesis. Labeling of pl-rRNA continues at the same rate in darkness after a brief light induction while cyt-rRNA labeling decreases 50% every 3 h in darkness after a brief light induction. Labeling of cyt-rRNA in wild type and W 3BUL is induced most effectively by blue light, green and red light being relatively ineffective. Labeling of pl-rRNA in wild-type cells is induced most effectively by blue light and red light, and the ratio of effectiveness of blue to red to green is the same as the ratios of absorption in these regions of the protochlorophyll(ide) holochrome from beans. These results are consistent with other data which indicate that the nonplastid aspects of plastid development are under control of a nonchloroplast blue light receptor while the plastid events are under the control of the plastid-localized protochlorophyll(ide) system.