To investigate whether the Ppr protein of R. centenaria participates in the chemotactic network, Ppr and, in particular, its histidine kinase GDC-0068 datasheet domain Pph were overexpressed in the chemotactic wild-type strain E. coli MM500. To this end, the plasmids pBAD-Ppr, pBAD-Pph and pBAD-PphH670A encoding the entire photoreceptor Ppr, the C-terminal histidine kinase domain Pph and the mutant PphH670A protein, respectively (Figure 1), were used to transform E. coli MM500. These plasmids carry the cloned genes under the CB-839 control of the arabinose-inducible

araBAD promoter. First, protein expression was analyzed by SDS-PAGE and Coomassie-blue staining. All three Ppr-derived proteins were expressed in the presence of arabinose (Figure 2A, even numbered lanes) but not in the presence of fructose (odd numbered lanes). Next, the chemotactic behaviour of the transformed cells find more was assayed. TB swarm agar plates, containing either arabinose or fructose were inoculated with the respective cells, incubated for 6 hours at 37°C and the swarm diameters were compared (Figure 2B). The chemotactic response of the wild type strain E. coli MM500 without or with the empty pBAD vector was clearly visible by the formation of a swarming ring (lower left and central panels).

The response was completely abolished when cells containing the plasmids pBAD-Ppr or pBAD-Pph were grown in the presence of arabinose. In these cases no swarm rings were visible (upper left and central panels). However, the expression of the mutant protein Pph-H670A N-acetylglucosamine-1-phosphate transferase where the histidine residue at position 670 has been substituted with an alanine residue, led to an only intermediate chemotactic response (upper right panel). The histidine residue at 670 of Pph

is a putative phosphorylation site and is located in a H-box region [29]. All strains were also analyzed on swarm plates containing 0.2% fructose that did not induce the expression of the Ppr proteins and did not significantly affect the size of the swarming rings (Figure 2B). As a control, the histidine kinase KdpE from R. centenaria was overexpressed which did not interfere with the chemotactic swarming (lower right panel). To rule out that the inhibitory effect on chemotaxis is caused by a reduced growth rate due to the heterologous expression of the Rhodocista proteins, growth curves of induced and non-induced and empty plasmid control cells were recorded and compared. No differences in growth rates depending on the presence of arabinose or fructose in the media were found (data not shown). Figure 1 Domain structure of the Ppr photosensor protein of R. centenaria . The Ppr protein consists of a photoactive yellow protein domain (Pyp; residues 1-135) which carries the blue light absorbing chromophore p-hydroxycinnamic acid, a central bacteriophytochrome bilin binding domain (residues 136-601) with the red light absorbing biliverdin chromophore, and a histidine kinase domain (Pph; residues 602-884).