INSERTION OF TELOMERE REPEAT SEQUENCE DECREASES PLASMID DNA CONDENSATION BY COBALT(III) HEXAAMMINE

Telomere repeat sequence (TRS) DNA is bound at the termini of most euk aryotic chromosomes. The sequences are highly repetitive and G-rich (e .g., [C(1-3)A/TG(1-3)](n) for the yeast Saccharomyces cerevisiae) and are packaged into nonnucleosomal protein-DNA structures in vivo. We ha ve used total intensity light scattering and electron microscopy to mo nitor the effects of yeast TRS inserts on in vitro DNA condensation by cobalt (III) hexaammine. Insertion of 72 bp of TRS into a 3.3-kb plas mid depresses condensation as seen by light scattering and results in a 22% decrease in condensate thickness as measured by electron microsc opy. Analysis of toroidal condensate dimensions suggests that the grow th stages of condensation are inhibited by the presence of a TRS inser t. The depression in total light scattering intensity is greater when the plasmid is linearized with the TRS at an end (39 - 49%) than when linearized with the TRS in the interior (18-22%). Circular dichroism o f a 95-bp fragment containing the TRS insert gives a spectrum that is intermediate between the A-form and B-form, and the anomalous condensa tion behavior of the TRS suggests a noncanonical DNA structure. We spe culate that under conditions in which the plasmid DNA condenses, the t elomeric insert assumes a helical geometry that is similar to the A-fo rm and is incompatible with packing into the otherwise B-form lattice of the condensate interior.