Varying competence for protein import into chloroplasts during the cell cycle in Chlamydomonas

Abstract

By studying the import of radioactively labelled small subunit of ribulose‐1,5‐bisphosphate carboxylase (pSS) into chloroplasts of the green alga C. reinhardtii cw‐15 protein delivery to chloroplasts was found to vary during the cell cycle. Chloroplasts were isolated from highly synchronous cultures at different time points during the cell cycle. When pSS was imported into ‘young’ chloroplasts isolated early in the light period about three times less pSS was processed to small subunit SS than in ‘mature’ chloroplasts from the middle of the light period. In ‘young’ chloroplasts also, less pSS was bound to the envelope surface. During the second half of the light period the import competence of isolated chloroplasts decreased again when based on chlorophyll content or cell volume, but did not change significantly when related to chloroplast number. Measurements of pSS binding to the surface of chloroplasts of different age indicated that the adaptation of protein import competence during the cell cycle is due to a variation of the number of binding sites per chloroplast surface area, rather than to modulation of the binding constant.

Differential Age-Dependent Import Regulation by Signal Peptides

“…Similar to results reported previously [16], import of Cr-prRBCS was highest in the 6-h chloroplasts and lower in the 1- and 11-h chloroplasts (Figure 5). All four other precursors tested showed the same pattern as Cr-prRBCS, despite the fact that they were recognized as group I (prOE23 and Cr-prRBCS) and group III precursors (prTic40, prL11, and Cr-prL11) by pea chloroplasts (Figures 1 and S5).…”

“…Each Chlamydomonas cell contains a single chloroplast. Furthermore, cultures of Chlamydomonas can be synchronized by light/dark cycles, and ages of chloroplasts can be defined by hours of culturing in the light [16]. Thus, Chlamydomonas also has an age spectrum of chloroplasts during development but does not face the problem of having leaves/chloroplasts of different ages supported on the same stem in the same organism.…”

Biogenic membranes of the chloroplast in Chlamydomonas reinhardtii

“…PCC 6803 and a GFP-tagged Tic20 paralogue which was seen to be localized to specific regions of the chloroplast envelope in A. thaliana (28,29). The effect of light on the relocalization of the TIC and TOC import machinery to lobe junctions might be relevant to the light stimulation of chloroplast protein import which has been observed in C. reinhardtii and vascular plants (24,30). Moreover, the Rubisco holoenzyme might be assembled in the pathways described here because its small subunit is imported via the TOC and TIC pathway and the chloroplast mRNA encoding the large subunit localizes in situ in the T-zone and is translated in association with membrane (7,31,32).…”

“…First, it is specific to import machinery because it was not observed for many other chloroplast proteins, the localization of which we have examined with this method (6,7,23). Second, this localization pattern may be physiologically relevant because the percentage of cells showing it dropped during incubation in the dark for 2 h, a condition associated with reduced rates of PSII biogenesis and chloroplast protein import in C. reinhardtii (6,24). When ML cells were dark-adapted immediately before fixation, the percentages showing localization around lobe junctions dropped from 48 to 11% for Toc75 (n = 74) and from 47 to 15% for Tic110 (n = 52).…”

Relationships between growth, development and photosynthetic activity during the cell cycle ofDesmodesmus armatus(Chlorophyta) in synchronous cultures

“…The assembly of functional chloroplast ribosomes depends on a supply of proteins produced in the cytoplasm (Gershoni & Ohad, 1980). For instance, chloroplast DNA polymerase and DNA-dependent RNA polymerase are coded in the nucleus and many essential chloroplast structures consist of supramolecular complexes involving both chloroplast and cytoplasmic translation products (Su et al, 2001). Moreover, chloroplast division seems to be under nuclear control and the number of daughter chloroplasts corresponds exactly to the number of daughter cells and to the size of cell.…”