Studies on the Folded Chromosome of E. coli

Abstract

There are a few obvious questions about the compact tridimensional structure of the DNA inside the bacterial cell. The first question is: How does the E. coli chromosome, which is one millimeter long, fold up inside the nuclear body of only one micron diameter? A second, related question is: How do the two daughter chromosomes separate from each other at the end of the replication cycle; how do they segregate into two daughter cells? We thought that the structure of the chromosome must be quite organized (not random), particularly after we were aware of one peculiar type of chromosome segregation which we could induce after producing numerous chromosome initiations in E. coli (multifork replication). It is possible to trigger the initiation of chromosome replication in E. coli by blocking DNA synthesis. If the block in DNA synthesis is short enough, after releasing the block the replicating fork will continue and a new fork will start at the chromosome origin. We found a few years ago (Worcel, 1970) that we could block and release using the temperature sensitive strain (DNA B) of E. coli which stopped DNA synthesis upon a shift to non-permissive temperature. Coming back to the permissive temperature we found that a new chromosome initiation was induced. We could repeat that process, shifting from 30 to 42 degrees and each time a new fork was initiated. We were able to induce 5 to 10 replication forks per chromosome and the cell didn’t seem to be incumbered by them. Upon return to the permissive temperature, the daughter chromosomes were eventually disentangled and became part of viable daughter cells.