Subunit control of Rubisco biosynthesis - a relic of an endosymbiotic past?

Subunits of multiprotein complexes in the chloroplasts of eukaryotic cells are frequently the products of protein synthesis in the nucleus-cytoplasm and the organelle. The mechanisms that integrate gene expression in the two compartments are poorly understood. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a model of nuclear-chloroplast interactions because it is a relatively simple example of a multimeric complex, being composed of nuclear DNA-encoded (RbcS) small subunits (SS) and chloroplast DNA-encoded (rbcL) large subunits (LS). One means by which RbcS and rbcL expression are coordinated is by the adjustment of subunit stoichiometries in response to the abundance of unassembled subunits. This type of integration occurs by two principal mechanisms. When SS accumulation is limiting (as in antisense mutants of tobacco), LS levels are primarily adjusted to those of the SS at the level of rbcL mRNA translation initiation. On the other hand, when LS accumulation is limiting (as in some rbcL nonsense and missense mutants), SS levels are adjusted to those of the LS at the level of protein degradation. These two mechanisms may be ubiquitous and serve as either fine-tune or course controls during normal growth and development. Autogenous control is a central theme of prokaryotic gene regulation, and intergenomic regulation of RbcS and rbcL expression by subunit concentrations may be a relic of an endosymbiotic past.