20 S.D. Levene
be significant bending of the DNA arms that emerge from the junction. In
the latter case at least 90 degrees of DNA bending would be needed per
recombination site to account for the observed discrepancy. Such strong bends
are not uncommon among the class of characterized DNA-bending proteins.
2.1.7 Chromatin and Recombination – Wrapping It All Up
As a footnote to the above discussion of the Flp recombination system, it
is worth asking how well this recombination system acts on DNA present as
chromatin in vivo. Studies by Stewart and colleagues show that Flp acts quite
efficiently on chromatin, generating a distribution of recombination products
that are consistent with a significant reduction in apparent DNA persistence
length [43]. The effect on apparent persistence length is expected because
of the approximately random bends induced in DNA by binding of histone
octamers. What is more surprising is that the system works at all, which
suggests that chromatin in vivo is a highly dynamic entity.
A mere 50 years have elapsed since the structure of the DNA double
helix was first elucidated [44]. It is probably fair to speculate that our un-
derstanding of DNA structure and dynamics far outstrips what Crick and
Watson might have imagined in 1953. Rather than being an inert repository
of genetic information, the DNA molecule is capable of taking on a wide range
of sequence-dependent structures, some of which exert profound effects on the
molecule’s accessibility and its participation in activities such as transcription,
replication, recombination, and repair. The genomic DNA of eukaryotes is a
particularly dynamic entity in vivo whose structure is subject to continual
modification through the displacement of nucleosomes and interactions with
chromatin-remodeling complexes. Since the 30,000 or so genes in the human
genome account for only a small fraction of the genome’s information content,
it is likely that there is much that remains to be learned about the structure
and function of the bulk of human DNA.
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