Abstract:
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BACKGROUND: Despite the profound current knowledge of the
architecture and dynamics of nucleosomes, little is known about
the structures generated by the interaction of histones with
single-stranded DNA (ssDNA), which is widely present during
replication and transcription. METHODS: Non-denaturing gel
electrophoresis, transmission electron microscopy, atomic force
microscopy, magnetic tweezers. RESULTS: Histones have a high
affinity for ssDNA at physiological salt concentrations, with an
apparent binding constant similar to that calculated for their
association with double-stranded DNA (dsDNA). The length of DNA
(number of nucleotides in ssDNA or base pairs in dsDNA)
associated with a fixed core histone mass is the same for both
ssDNA and dsDNA. Whereas histone-ssDNA complexes show a high
tendency to aggregate in 0.2 M NaCl, at lower ionic strength
nucleosome-like structures are formed. Core histones are able to
protect ssDNA from digestion by micrococcal nuclease, and a
shortening of ssDNA occurs upon its interaction with histones.
The purified (+) strand of a cloned DNA fragment of nucleosomal
origin has a higher affinity for histones than the purified
complementary (-) strand. CONCLUSIONS: At physiological ionic
strength histones have high affinity for ssDNA, possibly
associating with it into nucleosome-like structures. General
Significance In the cell nucleus histones may spontaneously
interact with ssDNA to facilitate their participation in the
replication and transcription of chromatin. |