Analysis of Promoters of Arabidopsis thaliana Divergent Gene Pair SERAT3;2 and IDH-III Shows SERAT3;2 Promoter is Nested Within the IDH-III Promoter.

Intergenic regions of divergent gene pairs show bidirectional promoter activity but whether regulatory sequences for gene expression in opposite directions are shared is not established. In this study, promoters of divergently arranged gene pair At4g35640-At4g35650 (SERAT3;2-IDH-III) of Arabidopsis thaliana were analyzed to identify overlapping regulatory regions. Both genes showed the highest expression in flower buds and flowers. 5' RACE experiments extended the intergenic region from 161 bp shown in TAIR annotation to 512 bp. GUS analysis of transgenic A. thaliana plants carrying the 691 bp fragment (512 bp intergenic region plus 5' UTR of both the genes) linked to uidA gene revealed that SERAT3;2 promoter drives gene expression in the tapetum, whereas IDH-III promoter functions specifically in microspores/pollen. Serial 5' deletion of the 691 bp fragment showed SERAT3;2 promoter extends up to -355 position, whereas IDH-III promoter encompasses the 512 bp intergenic region. In transgenics, uidA transcript levels were lower than native SERAT3;2 and IDH-III transcripts indicating presence of additional cis regulatory elements beyond the 691 bp fragment. The present study demonstrated for the first time occurrence of a nested promoter in plants and identified a novel bidirectional promoter capable of driving gene expression in tapetum and microspores/pollen.