Rice Researchers Uncover Mechanism of Bacterial DNA Separation

Researchers at Rice University have investigated the mechanism by which bacterial cells separate their DNA during binary fission. Unlike human cells that use mitosis, bacteria utilize binary fission, a faster process to separate circular chromosomes as they replicate.

José Onuchic, the Harry C. and Olga K. Wiess Chair of Physics at Rice University and a corresponding author of the study, noted that during binary fission, the daughter DNA strand separates from the mother strand simultaneously with its creation. This process relies on interactions between the two DNA copies.

Investigating the Role of SMC Proteins

The research team analyzed Hi-C maps, which illustrate the 3D structure of chromosomes, and combined this data with a physical modeling approach. Their focus was on the highly conserved structural maintenance of chromosomes (SMC) protein family.

Sumitabha Brahmachari, the first author and a research scientist in Onuchic's lab, stated that the goal was to understand how SMC drives DNA copy separation.

SMC Drives DNA Separation Through Compaction

By comparing chromosome models of bacteria with functional SMC proteins to those with defective SMC, researchers observed how chromosome structure changed during replication.

The findings indicated that SMC facilitates DNA separation through lengthwise compaction, generating a repulsive force between the two DNA copies. This process contributes to reliable DNA segregation.

During bacterial DNA replication, which starts at an origin of replication (ori) and proceeds simultaneously around the circular chromosome, SMC causes the replicating DNA copy to fold, similar to an accordion. This lengthwise compaction enhances the repulsion between the two DNA copies. As replication progresses, the increasing repulsion causes the replicating DNA copy to peel away from the original. By the completion of replication, the two oris are positioned on opposite sides of the cell, allowing for a clean division into two cells, each with a complete chromosome copy.

Consequences of Defective SMC

In the absence of SMC, the repulsive forces between the DNA copies are significantly weaker. Instead of lengthwise compaction and separation, the DNA copies adopt flexible, stringy configurations, with their oris remaining close.

This impedes clean DNA separation during cell division, potentially resulting in DNA damage or uneven chromosome distribution between daughter cells.

Future Research

Further research is planned to investigate how SMC enables this folding and to understand the stringy states observed without SMC.

Funding Support

The study received support from the National Science Foundation and the Welch Foundation.