Julia Williams, Top Lab Alum - Former Research Assistant, May 2007

…Research

Broad-host-range plasmids can transfer between and replicate in a wide range of bacteria. Plasmids of the IncP-1 group are considered the most promiscuous self-transmissible plasmids and are known to be stably maintained in the absence of selection. By coding for a variety of phenotypic traits they are important in the adaptation of their hosts to changing environments, e.g. new antibiotics or pollutants. We have previously shown that the IncP-1β 64,508bp multiresistance plasmid pB10 confers a small cost and is stably inherited in several diverse species. However, we found one environmental isolate, Pseudomonas putida strain H2, in which pB10 poses a cost of 20% and is rapidly lost in the population in the absence of selection. After experimental evolution of H2(pB10) for 1,000 generations with regular switching between isogenic hosts, plasmids were shown to confer a significantly lower cost to the ancestral host than the ancestral plasmid pB10. One of these evolved plasmid, named pME3, conferred only 1% fitness and also showed increased stability in the ancestral host. To understand the molecular basis for this phenotypic change, I determined the complete DNA sequence of plasmid pME3 and compared it to the completely annotated sequence of pB10. The results showed two mutations: D220A in TrfA, the replication initiation protein and a 254-base pair deletion of an orfE-like integron gene cassette of unknown function. I am now constructing plasmids that have only one mutation to understand the role of each mutation on the decreased cost and increased stability. Additionally, the copy number of pME3 relative to that of pB10 will be determined.

…Publications

Selection of publications in peer-reviewed journals

2008

De Gelder, L., J. J. Williams, J. Ponciano, M. Sota, and E.M. Top. 2008. Adaptive plasmid evolution results in host range expansion of a broad-host-range plasmid. Genetics. [pdf] In press.