Hypothetic model of distal promoter activity inhibition in double promoters.

<p><b>A</b>, <b>B</b> and <b>C</b> – hypothetic structures of double promoters PhST, PhTS and PhTSurv269, respectively. The constituent promoters PhSurv/PhSurv269 and PhTERT are denoted by grey and black lines, respectively. Small gray and black circles designate Sp1 transcription factors bound to Sp1 sites in the PhSurv/PhSurv269 and PhTERT promoters or in Sp1 multimers. Black dots with arrows indicate transcription start sites. −256 and −1456 in panel <b>B</b> delimit part of the PhSurv promoter sequence absent from PhSurv269. L 200, 1280 and 120 (bp) denote the length of the putative loop limited by positions of Sp1 sites. Sp1 sites are marked with letters A–G according to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046474#pone.0046474-Xu1" target="_blank">[14]</a>. Black and bold B, C, F and G letters denote the Sp1 sites chosen for mutagenesis. The activity of the distal promoter in the tandem is supposed to be inhibited due to formation of a DNA loop structure with transcription start sites of the distal promoter located inside the loop. In the PhTSurv269 promoter (panel <b>C</b>), the length of the putative loop (120 bp) is too small to allow its formation.