The core promoter of A3G is located within the region −114/+66 relative to the TSS
For characterization of the A3G promoter, we cloned the 1025 bp located at position −959/+66 relative to the identified transcription start into the promoterless pGL3-Basic luciferase reporter plasmid and designated the plasmid pGL3-Basic-APOprom1025. Similarly, pGL3-APOprom502 (containing sequence −436/+66), pGL3-APOprom225 (containing sequence −159/+66) and further 5′ deletion reporter constructs containing 180, 150, 120 or 60 bp upstream of position +65 were generated (Figure 3A). In order to analyze transcriptional activity, the luciferase reporter plasmids were transiently transfected into A3.01 T cells. Luciferase assays revealed a ∼20-fold increased transcriptional activity of the 1025 bp sequence as compared to the empty vector, indicating that we had identified an active A3G promoter sequence (Figure 3B). This transcription rate was not significantly altered by the 5′ deletions leading to the 502, 225 and 180 bp fragments (Figure 3B). In contrast, a drop in luciferase activity was observed in the case of the 150 bp fragment. This construct only retained 28% of the transcriptional activity of the 180 bp promoter in A3.01 T cells and activity of the 120 bp fragment was further reduced. Comparable reductions relative to the activity of 180 bp fragment were observed in the myeloid cell line U937 and the hepatic cell lines HepG2 and Huh7 (Figure 3C), indicating that the core promoter of A3G is located within the region −114/+66 relative to the TSS.
Figure 3. Luciferase activities of A3G promoter constructs in different cell lines. (A) A3G promoter 5′ deletion constructs of different sizes were cloned into pGL3-Basic luciferase reporter plasmids. Numbering is relative to the major TSS. A putative Sp1/Sp3 consensus site (gray square) is depicted. (B) A3.01 T cells were transiently transfected with the A3G promoter deletion constructs. Numbers on the x-axis refer to the length of the A3G promoter fragments in bp. (C) A3G promoter plasmids were transfected into U937, HepG2 and Huh7 cell lines. Numbers in the legends refer to the length of the A3G promoter fragments in bp. After 48 h, cells were harvested for luciferase assay. Firefly luciferase activities were normalized to coexpressed renilla luciferase activities. Mean values (±SD) of a representative experiment performed in triplicate are shown.