Rasika M. Harshey

We have two major research interests: Phage Mu and Bacterial Signaling & Motility. 

(1) Transposable Phage Mu and E. coli Genome Organization. DNA transposition is central to the propagation of phage Mu as it is to retroviruses. In the last several decades, we have analyzed the proteins and DNA sites that participate in transposition, discovered and characterized an enhancer element which is required for building a transpososome, deciphered the mechanism by which Mu integrates into its host upon phage infection, and showed that repair of the integrant is dependent on host replication complex (Pol III). (i) In recent years our attention has been focused on using our knowledge of Mu transposition to understand the organization of the E. coli host chromosome. This work has advanced a new view of the structure and dynamic properties of E. coli genome by showing that the chromosome is well-mixed and uncompartmentalized, with transpositions occurring freely between all measured loci. (ii) Mu amplifies its genome by replicative transposition, inserting repeatedly into the E. coli genome. However, its own genome is immune to insertion. We are trying to understand this phenomenon.

(2) cyclic-di-GMP and Necrosignaling. (i)Bacteria use rotation of helical flagella to propel themselves either through bulk liquid (swimming), or through a thin film of liquid on a solid surface (swarming). Moving over a surface presents challenges not found during swimming, the most important being lack of water. C-di-GMP signaling is known to disfavor motility by promoting the synthesis and secretion of polysaccharides that make biofilms. However, we are finding that polysaccharides play an important role as osmolytes in attracting water to the surface. Thus, on a surface, c-di-GMP is required for motility. (ii)  Swarming bacteria exhibit adaptive resistance to multiple antibiotics. Part of this resistance is due to antibiotic-induced death of a sub-population that releases a ‘necrosignal’ that stimulates efflux of the antibiotic from live cells. This phenomenon exists in both Gram-negative and Gram-positive bacteria and displays species-specificity. Given that adaptive resistance is a known incubator for evolving genetic resistance, our findings might be clinically relevant to the rise of multidrug resistance.