Rhodobacter capsulatus is a model organism for studying gene transfer agents (GTAs).
GTAs are a unique facilitator of gene transfer in prokaryotes. The DNA binding response
regulator CtrA plays a key role in modulating GTA activity in R. capsulatus, as well as flagellar
biosynthesis and cell motility. CtrA is an OmpR/PhoB response regulator with an N-terminal
receiver domain and a C-terminal transcriptional regulator domain. One unusual aspect of CtrA
function in R. capsulatus is that it regulates gene expression in both the phosphorylated and nonphosphorylated
forms. Using overlap extension PCR, the constructs for expression of three of
different versions of ctrA in R. capsulatus were prepared: wild type, phosphomimetic, nonphosphorylatable.
These constructs place the genes under the control of the R. capsulatus puf
promoter for high level of expression and the encoded proteins have 6×-histidine tags for
purification in studies aimed at determination of the DNA binding sites of the different versions
of CtrA. Horizontal gene transfer is an interesting way that bacteria can increase their genetic
diversity. In this work, the distribution of ctrA in the Alphaproteobacteria was examined and
evidence of horizontal gene transfer of this gene was found. Using phylogenetic analyses, several
instances of apparent misclassification of alphaproteobacteria to the wrong orders were found
and one candidate ctrA horizontal gene transfer event that may have occurred in an ancestral
bacterium that subsequently evolved into one lineage within the order Sphingomonadales was
found
