Light-Induced Increase in the Number and Activity of Ribosomes Bound to Pea Chloroplast Thylakoids in Vivo

Abstract
Within 8-10 min of illumination, chloroplast thylakoids of pea (P. sativum) became enriched 30-100% in ribosomes bound by nascent chains. Following (or, in some experiments, coincident with) this apparent redistribution was a 25-65% increase in the total bound ribosome population, which was then maintained at this higher level during the normal light period. On transfer of plants to darkness, the bound ribosome population decreased to the lower dark level. White, blue (400-520 nm) and orange (545-690 nm) light were all effective in producing an increase in the bound ribosome population. The level of bound ribosomes in the oldest leaves of 16-day-old plants was 15-fold less than in the still-maturing leaf but was increased by illumination. In vivo experiments with chloramphenicol and lincomycin, a requirement for protein synthesis by the 70S ribosomes for the light-induced shift to the population bound by nascent chains and for the increase in the total thylakoid-bound population was indicated. When thylakoids from plants in darkness or exposed to light for increasing periods were incubated in an Eschericia coli cell-free protein synthesizing system, 15 min of prior illumination in vivo produced a 60% increase in [3H]leucine incorporation. This stimulation preceded the increase in total bound ribosomes but corresponded in time to increases in the ribosomes bound by nascent chains. A light intensity of 100 .mu.mol/m2 per s, but not 25 .mu.mol/m2 per s, caused a significant increase in bound ribosomes over a 30-min period. Strong inhibition in vivo by 3'',4''-dichlorophenyl-1,1-dimethylurea suggests that noncyclic electron flow is essential for light-induced ribosome redistribution.