Nucleoprotein complexes present challenges to
genome stability by acting as potent blocks to replication.
One attractive model of how such conflicts
are resolved is direct targeting of blocked forks by
helicases with the ability to displace the blocking
protein-DNA complex. We show that Rep and UvrD
each promote movement of E. coli replisomes
blocked by nucleoprotein complexes in vitro, that
such an activity is required to clear protein blocks
(primarily transcription complexes) in vivo, and that
a polarity of translocation opposite that of the replicative
helicase is critical for this activity. However,
these two helicases are not equivalent. Rep but not
UvrD interacts physically and functionally with the
replicative helicase. In contrast, UvrD likely provides
a general means of protein-DNA complex turnover
during replication, repair, and recombination. Rep
and UvrD therefore provide two contrasting solutions
as to how organisms may promote replication of
protein-bound DNA.