Hin D restriction mapping of upaired regions in simian virus 40 superhelical DNA I: considerations regarding...

Superhelical simian virus 40 (SV40) DNA I was reacted with N-cyclohexyl-N'-beta-(4-methylmorpholinium)ethylcarbodiimide (CMC), and the location of CMC sites was mapped using the Hin D restriction endonuclease. The use of 14C-labeled CMC allows a quantitative analysis of the binding to the respective Hin D restriction endonuclease fragments. The percentage of reactivity was 6.54% for fragment A, 3.87% for fragment B, and 2.74% for fragment G. No CMC radioactivity was detected in other fragments. This reactivity is in agreement with the evaluation of binding by buoyant density measurements. The above fragments also contain the sites susceptible to S1 endonuclease action. This adds further support to the view that superhelical DNA can contain regions of localized interrupted secondary structure which may be capable of forming intrastrand hairpin structures if sequence relationships are favorable. The possible structure-function relationships for this model are discussed with the emphasis on transcription.