SeqA Protein Binding and the Escherichia coli Replication Fork

The membrane affinity of the SeqA protein tends to make it insoluble in whole-cell extracts unless the salt concentration is elevated. If SeqA protein binding forms a coherent filament of protein on the DNA with the sequences intervening between the GATC sites looped out, the newly replicated DNA would be organized and compacted as it emerges from the replication fork. The E. coli chromosome replication forks are not thought to move about the nucleoid as they progress around the chromosome. Chromosome segregation is a direct consequence of replication and occurs concomitantly with it. The DNA replication process itself drives DNA segregation, pushing the newly replicated DNA outward from the anchored replication forks toward opposite cell poles. The properties of the SeqA protein and its selective binding to the newly replicated DNA at the replication forks suggest that it might be directly involved in some or all of these processes. If the single-strand contact persisted through replication, newly replicated DNA would be uniquely marked by having protein subunits on one strand only. If this ‘‘hemidecorated’’ DNA were selfassociating, a mechanism similar to that proposed for SeqA might operate in the absence of any methylation signals.