DNA regions associated with the nuclear matrix of Ehrlich ascites cells expose single-stranded sites after...

Ehrlich ascites cells were pulse-labeled with [3H]thymidine and subjected to prolonged labeling with [14C]thymidine. The isolated nuclei were digested with the restriction endonuclease BspRI and then processed to yield a 'matrix fraction' and a 'non-matrix fraction'. The DNA fragments purified from these fractions and from whole digested nuclei were examined for nitrocellulose-binding sites before and after digestion with single-strand-specific (S1) nuclease. Both, pulse-labeled and long-time-labeled fragments, isolated from the matrix fraction, exhibited a significantly increased content of nitrocellulose-binding sites. The major portion of these sites were rendered non-binding by digestion with single-strand-specific nuclease and consisted most probably of structures exposing relatively small stretches of non-base-paired DNA. The nature of the minor portion of binding sites which was insensitive to single-strand-specific nuclease is not clear. Both types of binding sites are possible candidates for mediating the attachment of DNA to the nuclear matrix.