Our Science – Chattoraj Website
Dhruba K. Chattoraj, Ph.D.
Chromosome Maintenance in Bacteria
The faithful replication of chromosomes and their accurate segregation during cell division are required for the stable transmission of the genome. A host of intricate control mechanisms allow chromosome duplication only once per cell cycle, and ensure that a full complement of chromosomes is passed into each daughter cell. Defects in any aspect of chromosome replication or segregation pose significant threats to genome stability, and thus not surprisingly, underlie the development of many human diseases, foremost among them cancer.
While most bacterial genomes consist of a single circular chromosome, it is becoming clear from genome sequencing projects that about 10% of bacterial genomes are divided into multiple chromosomes. These bacteria provide a unique opportunity to probe the mechanisms involved in the maintenance of multiple chromosomes, a question particularly relevant to eukaryotes. We study how replication and segregation are controlled and coordinated among multiple chromosomes in the model organism, Vibrio cholerae. It is closely related to the paradigmatic model organism Escherichia coli, but has two chromosomes (chrI and chrII).
V. cholerae is also a potent human pathogen. A better understanding of the mechanisms operating in bacterial models will enhance our general understanding of chromosome replication and segregation, and can guide the rational development of therapeutics for many human diseases.
Cell cycle control of V. cholerae chrI replication
Replication of chrI appears to be controlled similarly to that of the E. coli chromosome. Both bacteria depend on Dam methylation for efficient initiation and for restricting initiation to once per cell cycle. V. cholerae appears to have novel factor(s) to control the methylation state of the origin. We are in the process of identifying such factors. It is expected that replication of the two Vibrio chromosomes are coordinated, and methylation is a likely process to mediate communication between the two chromosomes. The need for coordination might have necessitated a more involved control of chrI replication compared to that of the E. coli chromosome.
Cell cycle control of V. cholerae chrII replication
The highly conserved protein DnaA, which is also homologous to some of the ORC proteins that initiate replication in eukaryotes, controls the majority of bacterial chromosomes. Replication of chrII is controlled differently but has the salient features of bona fide chromosomal replication: initiation of replication once per cell cycle replication and at a particular time of the cell cycle. Studies on chrII thus are likely to provide a fresh perspective on the basic regulatory principles that govern chromosomal replication. We are determining the genetic basis of the regulation and, where identified, the mechanisms behind the function of the regulators.
The chrII initiator protein, RctB, as a target for chemotherapy
An added value of studying chrII replication is that it has the potential to find anti-Vibrio drugs. V. cholerae causes significant human suffering world-wide, as evidenced recently in Haiti. The initiator protein specific to chrII, RctB, is essential for Vibrio survival and is unique to the Vibrio family, offering a specific target for chemotherapy. The protein has several distinct functional forms, as it is remodeled both by molecular chaperones and upon binding to DNA. The opportunities are there to develop multiple inhibitors specific to the different functional forms of the protein. These considerations have prompted us to embark on a systematic structure-function analysis of the protein.
Inter-chromosome communication in V. cholerae
One of our interests in studying a multichromosome bacterium is the ability to ask: Do the chromosomes communicate to improve the stability of their maintenance? We are approaching the answer to this question by developing systems in which replication or segregation of one chromosome or the other is specifically blocked. When chrI replication is selectively blocked, both chrII replication and cell division are inhibited. Surprisingly, when chrII replication is blocked, replication and segregation of chrI and cell division remain unaffected. Apparently, there is no checkpoint to ensure completion of replication and segregation of both the chromosomes before cell division. It also appears that there has been a one-way adaptation of chrII to the maintenance of the main chromosome (chrI), and not the reverse. An understanding of chromosome coordination is expected to bear on the evolutionary process of chromosome acquisition and cooption.
Communication between replication and segregation in V. cholerae
The processes that contribute to chromosome segregation in bacteria are still poorly understood. In addition to the range of mechanisms proposed which includes chromosome replication, there are clearly additional mechanisms that anchor the centromere analog to the cell pole in some bacteria, including V. cholerae. We are specifically interested in studying the role of chromosome replication in the segregation process and how the centromeres are retained at the pole after they have segregated.
Most sequenced bacterial genomes have orthologs of plasmid partition genes (par genes). Studies in several bacteria indicate that par genes, in addition to their role in chromosome segregation, contribute to other cellular processes such as chromosome replication and cell division. We have found that one of the proteins encoded by par of chrI both promotes chrI segregation and regulates chrI replication. The results are strikingly similar to those found in Bacillus subtilis. It is remarkable that the replication phenotype of par has been retained or has independently evolved in convergent fashion in the two bacteria that are believed to have diverged more than a billion years ago. We are interested in determining whether par genes in general modulate chromosomal replication.
This page was last updated on 8/20/2013.