Cyclization of globular DNA. Implications for DNA-DNA interactions in vivo

The rate of cyclization of lambda DNA varies over more than 6 orders of mag nitude, from 3.2 x 10(-7) s(-1) to 2 s(-1), in a Tris-EDTA buffer as a func tion of spermidine concentration. This variation is strictly correlated wit h the conformation of the chain. The highest rates are obtained when the ch ain is collapsed into a dense globular state. The effective concentration o f the chain ends in the reaction is then 87 000-fold greater than in the ra ndom coil state. These results show that DNA globularity must be taken into account to understand biological processes involving intramolecular DNA-DN A interactions.