Gene for Small Subunit of Ribulose Bisphosphate Carboxylase - Dozens of genes in plants are affected by light and many of these regulated by phytochrome. The expression of phytochrome gene is depressed and that of RUBISCO (ribulose, 1,5 bisphosphate carboxylase/oxygenase) gene is stimulated by red light. RUBISCO is involved in CO2 fixation and is the most abundant enzyme on the earth.

It consists of two subunits, each encoded by a separate gene, the smaller subunit 'rbcS' encoded by a nuclear gene and the larger subunit 'rbcL' encoded by a chloroplast gene. Although, rbcS from different plants are fairly similar, small differences that exist, can be used to distinguish mRNA of a transgene from that of the host plant using specific probes.

This LRE also seems to be repetitive (with two repeats) and each repeat unit can mediate light responsiveness and organ specificity. However, it has also been shown that both repeats of LRE are required for full expression of the rbcS gene in youngest leaves of tobacco plants.

It has also been shown using transgenic plants, that a different sequence upstream to rbcS is responsible for age responsiveness. For example, it was shown that the region from -410 to -166 bp is essential for high level of expression in mature leaves, but not important for light responsive expression in leaves, for which the region -166 to -149 bp was most important.