Ribulose bisphosphate carboxylase consists of cytoplasmically synthesized "small" subunits and chloroplast-synthesized "large" subunits.Large subunits of ribulose bisphosphate carboxylase synthesized in vivo or in organello can be recovered from intact chloroplasts in the form of two different complexes with sedimentation coefficients of 7S and 29S. About one-third to one-half of the large subunits synthesized in isolated chloroplasts are found in the 7S complex, the remainder being found in the 29S complex. Upon prolonged illumination of the chloroplasts, newly synthesized large subunits accumulate in the 18S ribulose bisphosphate carboxylase molecule and disappear from both the 7S and the 29S large subunit complexes. The 29S complex undergoes an in vitro dissociation reaction and is not as stable as ribulose bisphosphate carboxylase.The data indicate that (a) the 7S large subunit complex is a chloroplast product, that (b) the 29S large subunit complex is labeled in vivo, that (c) each of these two complexes can account quantitatively for all the large subunits assembled into RuBPCase in organello, and that (d) excess large subunits are degraded in chloroplasts.Ribulose-1,5 -bisphosphate carboxylasc / oxygenase (E.C. 4.1.1.39) catalyzes the COs fixation step in photosynthetic carbon reduction and the cleavage of ribulose bisphosphate by oxygen in a key reaction in photorespiration (1). In higher plants and green algae, and in some photosynthetic bacteria, the enzyme consists of eight 55,000-dalton "large" subunits and eight -12,000-dalton "small" subunits. The large subunit bears the catalytic site and the small subunit is of unknown function. The fully assembled enzyme has a molecular weight of ~550,000 and a sedimentation coefficient of 18S (2). In higher plants and green algae both biochemical and genetic data have ftrraly established that the large subunit is synthesized in chloroplasts and that the small subunit is synthesized in the cytoplasm (3). The small subunit is taken into the chloroplasts in the form of a precursor polypeptide, which is cleaved by a soluble endoprotease within the chloroplast before integration into the 18S ribulose bisphosphate earboxylase holoenzyme (4).Free subunits of ribulose bisphosphate carboxylase have been detected in extracts of barley (5) or pea seedlings (6, 7) provided with radioactive amino acids. The large subunits behave as a dimer or heterodirner with a sedimentation coefficient of 7S, and the small subunits sediment at 3S. These free subunits turn over in vivo and appear to represent the subunit 20 pools from which ribulosc bisphosphate carboxylase is assembled (7). In isolated pea chloroplasts, however, newly synthesized large subunits were reported to accumulate in a 60@000-to 700,000-dalton "aggregate" together with a 60,000-dalton polypcptide. This polypcptide has been called the "large subunit binding protein" and no other function has been assigned to it (8). For reasons which will be discussed, we refer to this "aggregate" as a large subunit bindin...
ASBTRACT Ribulose-1,5-bisphosphate carboxylase [RuP2Case; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] is composed of eight small subunits (Mr,14,000) and eight large subunits (Mr, 55,000). Newly synthesized large subunits are associated with two complexes having sedimentation coefficients of 7 and 29 S. Assembly of RuP2Case occurs in isolated intact chloroplasts in the light but not in the dark. When extracts of chloroplasts are treated with ATP or GTP, RuP2Case assembly is accelerated while the 29S large subunit complex is maintained. In the presence of Mg2+, ATP brings about almost complete dissociation of the 29S complex, whereas GTP and a nonhydrolyzable analog of ATP are without effect. These results indicate the existence of a complex set of reactions involving nucleotides, Mg2+, and several putative intermediates in RuP2Case assembly. It is postulated that these reactions at least partly account for the light dependence of RuP2Case assembly. In particular, ATP and GTP promote the assembly of large subunits into RuP2Case.
In Escherichia coli, the ability to elicit a heat shock response depends on the htpR gene product. Previous work has shown that the HtpR protein serves as a sigma factor (if32) for RNA polymerase that specifically recognizes heat shock promoters (A. D. Grossman, J. W. Erickson, and C. A. Gross Cell 38:383-390, 1984). In the present study we showed that g32 synthesized in vitro could stimulate the expression of heat shock genes. The in vitro-synthesized &2 was found to be associated with RNA polymerase. In vivo-synthesized 32 was also associated with RNA polymerase, and this polymerase (Eor32) could be isolated free of the standard polymerase (Ef70). E&2 was more active than Eof70 with heat shock genes; however, non-heat-shock genes were not transcribed by E&32. The in vitro expression of the htpR gene required E{J70 but did not require Eff32.
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