Group leader: Dr. Miguel Ángel Cevallos Gaos

The continuity of life depends of two highly coordinated processes: the duplication of the genetic material (replication) and its accurate and equally distribution into the daughter cells (segregation). In bacteria, chromosomes and plasmids of low-copy number are transmitted from one generation to the other with amazing precision, but the molecular mechanisms underling this phenomenon remain poorly understood.

The genes involved in replication and segregation of low copy-number plasmids are organized at least in two modules: the first one embraces those genes controlling the initiation of plasmid replication, and the other contains genes encoding the components of an active machinery of segregation. Each one of these modules are subject of an independent and complex regulation that difficult their study. The exceptions are the plasmids belonging to the repABC family. In these plasmids both replication and partitioning genes are encoded within a single operon.

The repABC operons are present not only on large plasmids of low copy number of some a -proteobacteria such as Rhizobium, Mesorhizobium, Sinorhizobium, Agrobacterium, Rhodobacter, Oligotropha, Ruegeria, Nitrobacter and Paracoccus, but also in the chromosomes of Agrobacterium and Brucella.

The repABC basic replicons consist of an operon of three protein-encoding genes repA, repB, and repC, a par-site and a highly conserved small antisense RNA (ctRNA) gene located between repB and repC. The first two genes of the repABC operon encode proteins that show sequence and functional similarities to ParA and ParB, the proteins of the best-characterized segregational system of chromosomes and plasmids. RepA, by itself or along with RepB, has been implicated in the negative transcriptional regulation of the repABC operon. RepA has also been identified as a trans-incompatibility factor. RepC is essential for replication and for this reason was suggested to be the initiator protein. RepA and the ctRNA were found to be incompatibility determinants.

The par-site is a centromere-like sequence that can be located upstream to the repA gene, between repA and repB gene or downstream of repC. This sequence is the target of RepB, one of the components of the segregation machinery. It has been shown that the par site is also a strong incompatibility factor.

In our group we have four research lines:

  • We are interested two understand at the molecular level which are the mechanisms involved in the genetic regulation of the repABC operon.
  • We are also studying the elements involved in the segregation of the repABC plasmids. Their interactions and their roles in plasmid partitioning.
  • We are investigating the mechanisms of replication of these plasmids, with special emphasis in the replication initiation process.
  • We are interested in the mechanisms that determine the positions of the repABC plasmid within the cell and during the cell cycle.