P53 OLIGOMERIZATION AND DNA LOOPING ARE LINKED WITH TRANSCRIPTIONAL ACTIVATION

We examined the role of p53 oligomerization in DNA binding and in tran sactivation. By conventional electron microscopy (EM) and scanning tra nsmission EM, we find that wild-type tetramers contact 18-20 bp at sin gle or tandem 19 bp consensus sequences and also stack in apparent reg ister, tetramer on top of tetramer. Stacked tetramers link separated D NA binding sites with DNA loops. Interestingly, the p53(1-320) segment , which lacks the C-terminal tetramerization domain, binds DNA consens us sites as stacked oligomers. Although the truncated protein binds DN A with reduced efficiency, it nevertheless induces DNA looping by self -association. p53, therefore, has a C-terminal tetramerization domain that enhances DNA binding and a non-tetrameric oligomerization domain that stacks p53 at consensus sites and loops separated consensus sites via protein-protein interactions. Using model promoters, we demonstra te that wild-type and tetramerization-deficient p53s activate transcri ption well when tandem consensus sites are proximal to TATA sequences and poorly when tandem sites are distal, In the presence of proximal s ites, however, stimulation by distal sites increases 25-fold. Tetramer ization and stacking of tetramers, therefore, provide dual mechanisms to augment the number of p53 molecules available for activation throug h p53 response elements. DNA looping between separated response elemen ts further increases the concentration of local p53 by translocating d istally bound protein to the promoter.