BBMB Research Seminars

April 21, 2008
Dr. Scott Nelson
Department of Chemistry
Pennsylvania State University

"A Signaling Mechanism for Lagging Strand Polymerase Recycling During Bacteriophage T4 DNA Replication"

1414 Molecular Biology Buidling
4:40 p.m.

Abstract:

One problem faced by all DNA replication machines is how to simultaneously and coordinately replicate two anti-parallel DNA strands.  The trombone model for DNA replication physically links the leading and lagging strand polymerases with the lagging strand template forming a loop that allows replication of both strands to proceed in the same direction.   Leading strand synthesis is continuous whereas lagging strand synthesis is discontinuous, resulting in the formation of Okazaki fragments. Consequently, the lagging strand polymerase, although remaining associated with the replisome, must transiently release from the DNA substrate upon termination of each Okazaki fragment and recycle to the newly synthesized RNA primer.  The mechanism of recycling has been the subject of intense investigation with two competing models gaining substantial support.  In the collision model, the collision of the lagging strand polymerase into the 5’ end of the previous Okazaki fragment triggers RNA primer synthesis and lagging strand polymerase recycling.  In the signaling model, the lagging strand polymerase releases from the DNA as the result of one or more event(s) associated with primer synthesis.  Data from experiments that directly test the validity of the collision and signaling models will be presented.  The results indicate that collision with the 5’ end of the previous Okazaki fragment is not necessary and that instead, the loading of the clamp protein onto the newly synthesized primer signals the polymerase to recycle.  The signaling mechanism eliminates the possibility of abortive Okazaki fragments and likely provides a mechanism for the continuation of the replication fork beyond sites of DNA damage.