Dissection of 5' upstream sequences for selective expression of the Nicotiana plumbaginifolia rbcS-8B gene.

We have previously isolated and characterized a gene (rbcS-8B) from the wild-type species Nicotiana plumbaginifolia, encoding the small subunit of ribulose-1,5-bisphosphate carboxylase. Using transgenic N. plumbaginifolia as a host, we found that a 5' upstream region (-1038 to +32) of rbcS-8B contains all the sequences required for organ-specific and light-dependent expression of the gene. Here we report a detailed analysis of the 5' upstream region of rbcS-8B. Gene transfer experiments indicate that the region between -1038 to -102 contains at least two enhancer-like elements. The proximal element located between -312 and -102, confers organ-specific and light-inducible expression upon a reporter gene and closely resembles previously identified elements in other light-responsive plant genes. The distal element with novel characteristics is located between -1038 and -589. It can enhance expression from the cauliflower mosaic virus 35S promoter, but only when the -90 to -46 region of the 35S promoter is present. It confers on the heterologous promoter the organ-specificity of a typical rbcS gene but the enhanced transcription in leaves is insensitive to light. These experiments show for the first time that organ-specificity and light-responsiveness can be determined by separable cis-regulatory elements. The implications for the regulation of light-responsive genes are discussed.