Localization and conditional redundancy of regulatory elements in rbcS-3A, a pea gene encoding the small subunit of...

Expression of the pea rbcS-3A gene, one of a family of genes encoding the small subunit of ribulose-bisphosphate carboxylase [EC 4.1.1.39], is regulated by light and is restricted to chloroplast-containing cells. We analyzed the effects of light and development on rbcS-3A expression in transgenic plants. Two highly conserved sequences ("boxes" II and III) around nucleotide position -150 (relative to the transcription initiation site, +1) are required for rbcS-3A expression. The so-defined positive elements overlap with previously identified negative light-regulatory elements. In the case of box II, which has sequence similarity to the core enhancer motif of simian virus 40, a GG----CC transversion is sufficient to abolish expression. The effect of mutations in boxes II and III can only be measured when sequences upstream of -170 are removed, and because sequences both 5' and 3' of -170 can direct light-regulated and organ-specific expression. This implies that there is a redundancy of cis-acting elements in the 5' noncoding region of rbcS-3A. However, we show that the sequences upstream of -170 are dispensable only in the mature leaves of a green plant. In contrast, in the young, expanding leaves at the top of a green plant, as well as in seedlings, the distal elements are required for high-level expression. Therefore, redundancy is not absolute, and the requirements for rbcS-3A expression change during plant development.