期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1994
卷号：91
期号：5
页码：1898-1902
DOI：10.1073/pnas.91.5.1898
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Chemical probing of two predominantly alternating purine-pyrimidine d(CA/TG)n repeats led us to propose previously that in supercoiled plasmids these elements adopt a non-B-DNA structure distinct from that of Z-DNA formed by d(CG)n sequences. Here, we present further evidence supporting this contention. Reactivity with the conformation-sensitive reagent chloroacetaldehyde, which reacts with unpaired adenines and cytosines, was confined strictly to adenines in the d(CA/TG)n repeat. In contrast, only bases outside the d(CG)n repeat exhibited chloroacetaldehyde reactivity. Two-dimensional gel analysis of topoisomers containing d(CA/TG)n tracts with bases out of strict purine-pyrimidine alteration revealed multiple superhelical-dependent transitions to an alternative left-handed structure. Within individual plasmid molecules, these multiple transitions resulted from the stepwise conversion of contiguous segments of alternating purine-pyrimidine sequence, which are delimited by bases out of alternation, to the full-length alternative conformation. When the left-handed helices increased in length to include more bases out of alternation, the average helical pitch changed substantially to produce a less tightly wound left-handed helix. Overall, these data indicate that d(CA/TG)n tracts adopt a left-handed conformation significantly different from that of the canonical Z-DNA structure of d(CG)n sequences.
