One large subunit of ribulose 1,5-bisphosphate carboxylase oxygenase in Medicago, Spinacia and Nicotiana

An alfalfa rubisco small subunit homologue shares cis-acting elements with the regulatory sequences of the RbcS-3A gene from pea

A genomic clone of RbcS was isolated from an alfalfa (Medicago sativa L. cv. Apica) genomic library and characterized. Although this clone has structural features similar to a functional gene, the second exon is interrupted by a stop codon and thus is not fully translatable in the plant. Sequence analysis of the 5' and 3' noncoding regions of RbcSK-1A showed a high sequence homology to the flanking sequences of the RbcS-3A gene from pea. The regions of homology contain many important cis-regulatory elements shown to be essential for regulation of the RbcS-3A gene in pea. The promoter of this alfalfa rubisco clone was used in a translational fusion to test its ability to control the expression of the GUS reporter gene in an homologous nuclear background. High levels of GUS enzyme activity were recorded. These strong levels are comparable to some exceptionally high levels produced in other studies following the use of photosynthesis gene promoters in fusions with the GUS reporter gene.

Nuclear DNA regulates the level of ribulose 1,5-bisphosphate carboxylase oxygenase in Medicago sativa L

The response to selection for leaf proteins was studied during three selection cycles. Selection for high total nitrogen content showed 75% heritability, and the levels of both ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco) and cytoplasmic protein were strongly under nuclear DNA control. High and low protein content were correlated with chloroplast area. Although the amounts of nuclear DNA were similar, the ratio of Rubisco/DNA and chlorophyll/DNA changed during the selection process. It can be concluded that the levels of Rubisco achieved in mature plants of M. sativa are under nuclear DNA control. The possible involvement of small subunit (SSU) genes in controlling these levels is discussed.