4.1, prrA − mutant, and prrBCA − mutant bacteria grown under low-oxygen conditions. The spectra correspond to lysates of the strains indicated, and were generated using samples having equivalent concentrations of total protein (1.3 mg/ml). Details regarding the strains are provided in Table 1. The peaks near 420 nm in the spectra of the mutant strain samples

can be attributed to cytochrome Soret bands, mostly obscured in the spectrum of the wild type 2.4.1 sample Ultrastructure of R. sphaeroides wild type 2.4.1, ppsR mutant, and ppsRprrA mutant membranes PpsR has been called a “master” regulator of photosystem development (Moskvin et al. LXH254 supplier 2005), and disabling ppsR leads to the expression of photosynthesis genes in the presence of oxygen. Thus, cells lacking PpsR are genetically extremely unstable under aerobic conditions (Alisertib purchase Gomelsky and Kaplan 1997). The activity of PpsR is controlled by interactions with the anti-repressor protein AppA (reviewed in Gomelsky and Zeilstra-Ryalls 2013). Recent studies have shown that transcription of the appA gene is PrrA-dependent. They also indicate that PrrA appears

to affect interactions between AppA and PpsR, which in turn influences the activity of PpsR. The consequences of this regulatory see more complexity are made apparent by virtue of the fact that, although phototrophic growth is abolished in prrA null mutant bacteria, bacteria lacking both PrrA and PpsR can grow phototrophically (Gomelsky et al. 2008). The status of either ppsR − or ppsR − prrA − mutant bacteria with respect to ICM formation has not been directly determined. In order to do so, TEM was used to examine the ultrastructure of cells grown under inducing anaerobic (dark) conditions that do not exert selective pressure for suppressor mutations that compensate for the absence of PpsR. ICM formation was apparently not affected by the absence of Urease PpsR, as the ultrastructure of the PPS1 (Table 1) mutant cell membrane appears similar to that of wild type bacteria (Fig. 3). This was to be expected, since PpsR functions as a repressor of PS genes under aerobic conditions, and ppsR null mutant bacteria grow normally under phototrophic conditions. Fig. 3 TEM of R. sphaeroides wild type 2.4.1,

ppsR − mutant, and prrA − ppsR − mutant bacteria that had been cultured under anaerobic–dark conditions with DMSO as alternate electron acceptor. The strains used are as explained in the legends, and details are provided in Table 1 Since PrrA is thought to be necessary for the inactivation of PpsR (Moskvin et al. 2005; Gomelsky et al. 2008), the ppsR − prrA − double mutant strain RPS1 (Table 1) should have normal ICM. However, long, tubular-shaped ICM was found to be a prominent feature of the cells (Fig. 3). Evidently, despite the abnormal appearance of the ICM, the photosynthesis machinery is nevertheless at least somewhat operational as the cells can grow phototrophically, although their growth is considerably slower than wild type (Moskvin et al. 2005).