Sequence-specific binding of protein factors to two independent promoter regions of the acidic tobacco pathogenesis-related-1 protein gene (PR-1)

Cloning and identification of the promoter of the tobacco Sar8.2b gene, a gene involved in systemic acquired resistance

Expression of the Sar8.2 gene family is induced by salicylic acid (SA) in tobacco during induction of systemic acquired resistance. Expression of Sar8.2b, one member of this 12-member family, was detected as early as 12 h after treatment with SA and was maximal 36 h after SA treatment. In NahG transgenic tobacco plants, benzothiadiazole and dichloroisonicotinic acid induced expression of Sar8.2b but SA did not, suggesting that expression of the Sar8.2b gene is SA-dependent. Several putative cis-acting elements were found in the Sar8.2b gene promoter region, including an as-1 element and GT-1 and Dof binding sequences. We constructed a series of progressive deletion mutations in the Sar8.2b promoter region linked to the beta-glucuronidase (GUS) coding region and analyzed GUS activities by stable expression in transformants of Arabidopsis thaliana. Deletions between -728 and -927 bp or between -351 and -197 bp of the promoter region resulted in a significant reduction in GUS activity induced by SA treatment as shown in stable transformants of A. thaliana. The -197 bp fragment of the promoter region was found to confer a relatively low level of GUS activity induced by SA treatment in stable expression of transformants in A. thaliana. The results suggest that 927 bp of the Sar8.2b gene promoter confers full promoter activity and that cis-acting elements required for high-level SA-inducible expression of the Sar8.2b gene may exist within the regions -728 to -927 bp and -197 to -351 bp.

Cloning and DNA-binding properties of a tobacco Ethylene-Insensitive3 (EIN3) homolog

Ethylene-Insensitive3 (EIN3) is a transcription factor that works in the ethylene signaling pathway in Arabidopsis. We isolated a tobacco cDNA encoding an EIN3 homolog as a sequence-specific DNA-binding protein. The encoded protein TEIL (tobaccoEIN3-like) shares 60% identity in amino acid sequence with EIN3. The DNA-binding domain was localized in the N-terminal half, which shows 92% identity in amino acid sequence with the corresponding region of EIN3, suggesting a conserved function in DNA-binding specificity. TEIL was indeed functionally similar to EIN3 because, like EIN3-overexpressing plants, transgenic Arabidopsis seedlings overexpressing TEIL cDNA exhibited constitutive triple response phenotypes. Random binding site selection analysis revealed that the consensus binding sequence for TEIL is AYGWAYCT, where Y and W represent A or C and A or T, respectively. A reporter plasmid containing the TEIL binding sites showed a 7- to 10-fold higher activation relative to that containing a mutated TEIL-binding sequence in tobacco protoplasts. A further 2- to 3-fold increase in activation was observed when a plasmid for TEIL overproduction was co-transfected, indicating that TEIL is a transcriptional activator. Moreover, nuclear extracts from ethylene-treated leaves showed an increase in DNA-binding activity specific to the TEIL-binding sequence, despite the level of the transcripts being unchanged. These observations suggest that TEIL functions as a transcription activator with a relatively redundant DNA-binding specificity, and its function may be regulated at least in part by modulation of the DNA-binding activity through ethylene signaling.

The PR-1a promoter contains a number of elements that bind GT-1-like nuclear factors with different affinity

The 900 bp promoter region of the tobacco PR-1a gene was divided into eight fragments using PCR. The fragments were tested for their ability to bind to nuclear factors isolated from tobacco leaf. Band shift assays demonstrated that all but one of the fragments specifically interacted with nuclear proteins. From competition experiments it was determined that the same nuclear factors bind various promoter fragments with different affinity. Moreover, efficient competition with a synthetic tetramer of box II of the rbcS promoter indicated that GT-1-like nuclear factors are involved in these interactions. Furthermore, in comparison to extracts from untreated plants, nuclear protein preparations from tobacco mosaic virus-infected tobacco showed a reduced GT-1 binding activity. These results will be discussed in relation to induced PR-1a gene expression.