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

Abstract

Some of the chloroplast 70S ribosomes are seen in electron micrographs to be associated with the thylakoid membranes (5, 9, 19, 28). At least some of these ribosomes must be associated with mRNA, since washed thylakoids are capable of synthesizing proteins when supplemented with soluble factors (1,2,8,20). Retention in the thylakoids of the newly synthesized peptides provided evidence that the proteins synthesized were discharged into the membranes (2,20). From this, it seemed likely that the ribosomes active in vitro were tightly bound to the thylakoids by a nascent peptide chain. The presence of this type of linkage was demonstrated more directly for Chlamydomonas (5) and for pea (28), using puromycin-induced release of the ribosomes in the presence of high salt concentrations (0.5 to 1.0 M KCI) as a criterion.The nascent peptide linkages between ribosomes and the thy-'

Differential Transcription of Pea Chloroplast Genes during Light-Induced Leaf Development (Continuous Far-Red Light Activates Chloroplast Transcription)

“…This change is particularly dramatic in the case of 16s rRNA, which is normally a very stable RNA (Kim et al, 1993). The decline of pea ribosomes during light-induced chloroplast development in pea was previously reported (Fish and Jagendorf, 1982). Most plastid proteins are very stable; therefore, there is little reason to maintain the full capacity to synthesize plastid proteins once leaf and chloroplast development is complete.…”

Further Characterization of Ribosome Binding to Thylakoid Membranes

“…While light in vivo can cause a net 25 to 75% increase in thylakoid-bound polysomes in pea chloroplasts (9), in most of our experiments in organello illumination only slowed down the extent of RNA loss during incubation (Table I). This was true for chloroplasts taken from plant in the light or in the dark.…”

“…In previous work (9) it was found that 40 to 70% more RNA (the indicator for 70S ribosomes) was tightly associated with thylakoids isolated from pea plants in the light, than from plants in the dark. This was demonstrated to be a direct effect of light, preventable by DCMU, saturated at a very low light intensity.…”

“…A more general piece ofevidence would 9) were grown for 7 d and had their chloroplasts isolated as described (19). The terms in the text: 'light' and 'dark' plants, and chloroplasts taken from them, refer to illumination (or lack of it) 1 to 2 h into the usual photoperiod.…”

Light regulation of protein synthesis factor EF-G in pea chloroplasts

“…In Euglena gracilis, EF-G is much more strongly light-regulated than EF-Tu [6], perhaps because many more copies of EF-Tu are required than EF-G. Fish and Jagendorf [9] noted an increase in puromycin sensitivity of thylakoid-bound ribosomes isolated from darkgrown pea seedlings exposed to a brief period (5 min) of illumination. This result is consistent with the notion that EF-G may be rate-limiting in dark-adapted chloroplasts, as nascent polypeptides would not be released by puromycin in the absence of translocation.…”