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Cell cycle and developmental control of chromosomal replication. We wish to understand how chromosome replication is coordinated with cellular growth and development so that replication occurs during the appropriate period of the cell cycle, and in the appropriate cell-type during cellular differentiation. The bacterium Caulobacter crescentus exemplifies this fundamental control problem, because its cellular differentiation is an obligate part of its cell division cycle, and chromosome replication does not initiate until the "swarmer" cell-type differentiates into the "stalked" cell-type. In both C. crescentus and E. coli, chromosome replication begins at a unique place, the origin of replication. However, despite the detailed knowledge of the biochemical interactions that take place at the E. coli origin, very little is actually known about the molecular mechanisms that couple replication to the cell cycle in this or any other bacterium. Presumably, special proteins monitor growth and relay the appropriate signals to the replication proteins at the origin of replication. 

In C. crescentus, where the cell cycle is more amenable to analysis, we propose that a key signaling protein (CtrA) couples replication with the cell cycle. This hypothesis is based on the CtrA protein's homology with a class of phosphorelay proteins (the OmprR response regulators) whose biochemical activities are altered by special phosphokinase proteins, and our recent results demonstrating that CtrA protein has five binding sites inside the C. crescentus origin of replication (Cori). Interestingly, Cori CtrA binding sites overlap an essential RNA polymerase promoter and an essential DnaA protein binding site, implying that CtrA may directly interact with these proteins that are thought to be the earliest acting proteins at the E. coli origin of replication. Despite the ubiquity of phosphorelay proteins, and the universal requirement for replication control, CtrA is the first candidate response regulator with a direct role in replication control. 

We are currently testing the hypothesis that CtrA is a major regulator of chromosome replication by employing several molecular genetic and biochemical approaches, and we are particularly interested in understanding how CtrA interacts with other proteins such as RNA polymerase and DnaA. Since the basic problem is one of reprogramming the chromosomes so that one chromosome is active and the other one is inactive in the appropriate cell-type, we are also studying proteins that modify DNA structure. In particular, we are studying a unique DNA methylation protein (that is also regulated by CtrA), and a new class of proteins that condense and decondense chromatin.