Tactic Responses to Oxygen in the Phototrophic Bacterium Rhodobacter sphaeroides WS8N

Abstract

The temporal and spatial behavior of a number of mutants of the photosynthetic, facultative anaerobe Rhodobacter sphaeroides to both step changes and to gradients of oxygen was analyzed. Wild-type cells, grown under a range of conditions, showed microaerophilic behavior, accumulating in a 1.3-mm band about 1.3 mm from the meniscus of capillaries. Evidence suggests this is the result of two signaling pathways. The strength of any response depended on the growth and incubation conditions. Deletion of either the complete chemosensory operons 1 and 2 plus the response regulator genes cheY 4 and cheY 5 or cheA 2 alone led to the loss of all aerotactic responses, although the cells still swam normally. The Prr system of R. sphaeroides responds to electron flow through the alternative high-affinity cytochrome oxidase, cbb 3 , controlling expression of a wide range of metabolic pathways. Mutants with deletions of either the complete Prr operon or the histidine kinase, PrrB, accumulated up to the meniscus but still formed a thick band 1.3 mm from the aerobic interface. This indicates that the negative aerotactic response to high oxygen levels depends on PrrB, but the mutant cells still retain the positive response. Tethered PrrB ؊ cells also showed no response to a step-down in oxygen concentration, although those with deletions of the whole operon showed some response. In gradients of oxygen where the concentration was reduced at 0.4 M/s, tethered wild-type cells showed two different phases of response, with an increase in stopping frequency when the oxygen concentration fell from 80 to 50% dissolved oxygen and a decrease in stopping at 50 to 20% dissolved oxygen, with cells returning to their normal stopping frequency in 0% oxygen. PrrB and CheA 2 mutants showed no response, while PrrCBA mutants still showed some response.Tactic responses are a common feature of motile bacteria. A sensory system converts environmental signals into a change in the direction of rotation of the flagellar motor, resulting in the accumulation of microbial populations in microenvironments optimal for growth (1).Taxis towards oxygen (aerotaxis) has been identified in several bacterial species (45). It allows bacteria to migrate to concentrations of dissolved oxygen ideal for their current metabolism. In many species investigated, an active respiratory chain and Che proteins, such as CheA, CheW, and CheY (20,21,35), are essential for responses. Recent reports have identified a methyl-accepting chemotaxis protein (MCP)-like oxygen receptor, Aer, in Escherichia coli (2, 31) and Pseudomonas putida (26), whereas myoglobin-like aerotaxis transducers have been identified in Halobacterium salinarum and Bacillus subtilis (16) and putative oxygen transducers with a c-type heme-sensing domain have been identified in other species (4, 8), suggesting that several independent systems have evolved to control tactic behavior in response to changes in oxygen level.Rhodobacter sphaeroides is a facultative non-sulfur-photosynthetic bacterium belonging to the ␣-...

Effects of Oxygen and Light Intensity on Transcriptome Expression in Rhodobacter sphaeroides 2.4.1

“…one gene at a time. With the advent of the DNA sequence, assembly and annotation of the R. sphaeroides 2.4.1 genome (21), as well as the construction of an Affymetrix GeneChip, we are in the unique position of being able not only to validate the earlier described observations, but more importantly we are in the position to extend these observations to the expression of all genes comprising the photosynthesis regulon including N 2 and CO 2 fixation, and taxis (22)(23)(24)(25), as well as other physiologic activities for which genetic elements are regulated by oxygen and light intensity. Finally, we are in the position of being able to follow the expression of well studied regulatory elements controlling the photosynthesis lifestyle, as these respond to oxygen-and lightinduced regulation.…”

Digging deeper: uncovering genetic loci which modulate photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1

“…This system is required for the activation of many PS genes and also participates in the control of genes involved in CO 2 , N 2 and H 2 utilization, as well as of those genes encoding elements of the respiratory electron transport chain (Joshi & Tabita, 1996;Qian & Tabita, 1996;Elsen et al, 2000;Swem et al, 2001). Recently, it was reported that the PrrBA system also positively regulates the che operon 2 of R. sphaeroides (Romagnoli et al, 2002). The PrrBA twocomponent system consists of the membrane-associated PrrB sensory histidine kinase and its cognate PrrA response regulator.…”

The Flagellar Set Fla2 in Rhodobacter sphaeroides Is Controlled by the CckA Pathway and Is Repressed by Organic Acids and the Expression of Fla1

“…In the presence of oxygen, RegB is inactivated and the synthesis of these complexes and proteins is halted. The absence of RegB has a negative effect on the growth rate under photoheterotrophic conditions (30), and these mutant cells do not show the negative tactic response toward high concentrations of oxygen (25).…”

“…Fla1-dependent taxis responds to several stimuli, some of which are not directly sensed by the chemotactic proteins. For example, it has been observed that aerotaxis is controlled in part by the two-component system RegBRegA (also known as PrrB and PrrA) (25). These proteins form a global system that responds to the redox state of the cell to control several metabolic processes that generate energy, i.e., photosynthesis, carbon and nitrogen fixation, aerobic and anaerobic respiration, electron transport, etc.…”