The Biosynthesis of Ribulose Bisphosphate Carboxylase

The quantitative relationships between ribulose bisphosphate carboxylase, nuclear ploidy, and plastid DNA content were examined in the nonisogenic polyploid series Triticum monococcum (2X), Triticum dicoccum (4x), and Triticum aestivum (6x). Ribulose bisphosphate carboxylase per mesophyl cel increased in step with each increase in nuclear ploidy so the ratios of ribulose bisphosphate carboxylase per mesophyUl ceUl (picograms) to nuclear DNA per mesophyUl ceUl (picograms) were almost identical in the three species. Ribulose bisphosphate carboxylase per plastid was 14.1, 14.7, and 16.8 picograms in the 2x, 4x, and 6x ploidy levels, respectively. Plastid area in these three species decreased with increasing nuclear ploidy so the concentration of ribulose bisphosphate carboxylase in the plastoids was 60% higher in the hexaploid compared to the diploid species. DNA levels per plastid were 64 and 67 femtograms for the diploid and tetraploid species, respectively, but were 40% less in the plastids of the hexaploid species. These relationships are discussed in terms of ceUlular and plastid control of ribulose bisphosphate carboxylase content.Increased nuclear ploidy levels in cereals and other plants are frequently correlated with changes in several cellular and chloroplast features (5-8, 11, 12, 16, 19, 24-26). One chloroplast protein which has received attention in this respect is RuBPCase.2 Although earlier reports suggested that the affinity of RuBPCase for CO2 increases with an increase in nuclear ploidy (14,21), later more-detailed examinations have failed to confirm this correlation (15,18). In recent work on a ploidy series of Festuca arundinacea (tall fescue) (15), it was found that the amount of RuBPCase, expressed as a concentration per leaf section, increased with ploidy from 4x to 8x. The changes were small but were also correlated with changes in net photosynthetic rate. If this relationship is found to be general for other species, the control of the synthesis of the amount of RuBPCase assumes a new importance. One current hypothesis proposes that the small subunit controls the intrachloroplast synthesis of the large subunit (2, 13). Experimental support for this hypothesis has come from our recent demonstration (10) that the two subunits undergo coordinated, simultaneous synthesis in young wheat leaves (Triticum aestivum) and that changes in the synthesis of the subunits can be accounted for by changes in their translatable mRNAs at each stage of devel- YOJ 5DD EnglandThe next question concerns the role of gene dosage in determining amounts of RuBPCase. There is limited experimental evidence for gene dosage effects on the synthesis of specific proteins in higher plants (15,19,25,26) although such effects are well documented in Drosophila in which the phenomenon is so well understood that dosage relationships have been utilized to map the chromosomal locations of genes which encode for specific enzymic activities (23).The availability of a polyploid series in the genus Triticum offered us an o...

Section: Discussionsupporting

confidence: 76%

“…If this relationship is found to be general for other species, the control of the synthesis of the amount of RuBPCase assumes a new importance. One current hypothesis proposes that the small subunit controls the intrachloroplast synthesis of the large subunit (2,13 …”

mentioning

confidence: 99%

Genome Expression during Normal Leaf Development

Dean

Leech²

1982

“…Studies with Chiamydomonas indicated coupling between the synthesis of each subunit and the complementary translation system ( 12). In contrast, results with higher plants such as barley (6), pea (5), and rye (10) seedlings, spinach protoplasts (18), and soybean leaf cells (2) suggested either coupling (12,18), partial coupling (6), or no coupling (2,10). These varied results may be due in part to the reliance of these studies on protein synthesis inhibitors, whether chemical (2,5,6,12,18) or environmental (10).…”

contrasting

confidence: 41%

Ribulose Bisphosphate Carboxylase Synthesis in Barley Leaves

Nivison

Stocking²

1983

ABSTRACIThe coordination of the synthesis of the large and small subunits of ribulose 1,5-bisphosphate carboxylase (RuBPCase) was studied in young light-grown barley (Hordeum vulgare L. var. UC566) leaves. Since a barley leaf is a continuum of different aged cells with the youngest cells at the base and the oldest at the tip, developmental changes could be investigated by comparing different leaf regions. The rate of total cytoplasmic protein synthesis increased to a maximum before the rate of total orpnelle protein synthesis. The different positions of the maxima suggested that the synthesis of the small RuBPCase subunit on cytoplasmic ribosomes and the large RuBPCase subunit on chloroplast ribosomes might not be coupled during barley leaf development. However, measurements of the amounts and rates of synthesis of the subunits showed that they were coupled. Although the amounts of the RuBPCase subunits increased from the younger to the older leaf regions, the subunits were present in an equimolar ratio. While the rates of synthesis of both subunits increased to a maximum in a midleaf region and then declined, the ratio of the rates remained constant. That the subunit amounts remained equimolar and the synthetic rates proportional while total RuBPCase synthesis was changing indicated that the synthesis of the subunits was closely coordinated during leaf development. A close coordination was also supported by the kinetics of the inhibition of subunit synthesis in the presence of cycloheximide.Ribulose 1,5-bisphosphate carboxylase is composed of large and small subunits which are synthesized in the chloroplast and cytoplasm, respectively, then brought together and assembled into a holoenzyme within the chloroplast (6, 23). The two types of subunits occur in the holoenzyme in equal numbers (13), and free subunits are either absent or present in relatively small amounts ( 15, 21

“…Since all of these studies monitored the presence or synthesis of both the large and the small subunits by radioactively labeling of these polypeptides with [35S]methionine, these results can not be directly compared to our observations which show a net excess in the molar quantity of the small subunit of RuBPCase in the dark. It is clear from this work and that of others (1,7,14) that there must be a different means of regulation for the expression of the small and large subunits of RuBPCase. One possible explanation for the disproportionate increase in the quantity of large subunit relative to the small subunit is that there is a significant increase in the number of genes for the large subunit during greening of etiolated leaf tissue.…”

Section: Discussionmentioning

confidence: 93%

Immunological Determination of Phosphoenolpyruvate Carboxylase and the Large and Small Subunits of Ribulose 1,5-Bisphosphate Carboxylase in Leaves of the C₄ Plant Pearl Millet

Bassett

Rinehart²,

Rawson³

1985

The light-dependent development of the photosynthetic apparatus in the first leaf of the CG plant pearl millet (Pennisetum americanum) was monitored by immunologically determining the concentration of phosphoenolpyruvate carboxylase and ribulose 1,5-bisphosphate carboxylase. A competitive enzyme-linked immunosorbent assay procedure using antibodies to the monomeric subunit of phosphoenolpyruvate carboxylase and the large and small subunit of ribulose 1,5-bisphosphate carboxylase was used to quantitate the amounts of these polypeptides in the first leaf of etiolated seedlings and etiolated seedlings exposed to light for varying periods of time. Phosphoenolpyruvate carboxylase was present in etiolated tissue; however, light stimulated its synthesis nearly 23-fold. Maximum accumulation of phosphoenolpyruvate carboxylase occurred approximately 4 days after etiolated plants were placed in the light. Both the large subunit and the small subunit of ribulose 1,5-bisphosphate carboxyhse were present in leaves of etiolated seedlings. Light also stimulated the synthesis of both of these polypeptides, but at different rates. In etiolated leaves there was approximately a 3-fold molar excess of the small subunit to large subunit. Exposure of the etiolated leaves to light resulted in the molar ratio of the large subunit to the small subunit increasing to approximately 0.72. These data indicate that the net synthesis of these two polypeptides is not coordinately regulated at all times.The effect of light on the photomorphogenesis of plants is well documented (4). Numerous morphological and biochemical studies indicate that the control of the expression of genes involved in the development of a fully photosynthetically competent plant may be at the level of transcription, translation, and/or enzymic activation (5,19,22 We have chosen to make such measurements in the first leaf of the C4 plant pearl millet (Pennisetum americanum) (8) using polyclonal antibodies to the monomeric subunit of PEPCase and the large and small subunit of RuBPCase in a competitive ELISA (2,9,16). This information should permit one to determine the length of time required for the light-dependent photomorphogenic development of a leaf of a C4 plant. MATERIALS AND METHODSGrowth of Plant Material. Pearl millet (P. americanum, Tift 23DB) seed (6) was planted in flats of Perlite, germinated, and kept in the dark at 25°C for varying periods of time. Etiolated plants that were to undergo greening were kept in the dark until the first leaf was completely unfurled (4 d