Bacteriophytochrome-Dependent Regulation of Light-Harvesting Complexes in Rhodopseudomonas palustris Anaerobic Cultures

Abstract

Bacteriophytochromes (Bphs) are photoreceptors that help bacteria sense changes in light wavelength and intensity. Bphs contain a linear tetrapyrrole chromophore that, upon absorption of red or far-red light, undergoes a cis-trans isomerization that leads to a conformational change in the holoprotein. The conformation and type of Bph affects the expression of genes. The linear tetrapyrrole bound by Bphs is thought to come from O(2)-dependent cleavage of heme by a heme oxygenase. We have discovered that the absence of O(2) does not inhibit the normal function of two Bphs in the regulation of Rhodopseudomonas palustris light-harvesting complexes. These observations imply that: (i) a linear tetrapyrrole can be made anaerobically, either through anaerobic heme cleavage by a novel enzyme or directly from the heme precursor hydroxymethylbilane without ring cleavage; or (ii) that Bph-dependent signal transduction does not require a chromophore.

Apo-bacteriophytochromes modulate bacterial photosynthesis in response to low light

“…The LH4 complex predominates when this ratio is >1, and the LH2 complex predominates when the ratio is <1 (16,19). We found that deletion of either one of the RpBphP genes disrupted LH4 synthesis in response to low light intensity, similar to previous reports (7,8) (Fig. 1B and Table 1).…”

“…Extensive in vitro work has shown that RpBphP2 and RpBphP3 can assemble with BV and function as light-regulated histidine kinases (7); however, because BV synthesis requires oxygen, it was thought that these RpBphPs might not always be associated with BV in vivo, particularly under anaerobic photosynthetic growth conditions (8,15,20,29). We found that RpBphP2 is bound to BV produced by HmuO in vivo in cells grown under extremely oxygenlimited conditions.…”

“…Apo-Phytochrome RpBphP2 and RpBphP3 Are Required to Sense Light Intensity. Previous work has shown that enhancement of LH4 synthesis occurs in response to decreasing intensity of white light under anaerobic conditions (8). Along with its role in sensing light quality, BV also may play a role in sensing light intensity.…”

“…Two R. palustris BphPs, designated RpBphP2 and RpBphP3, function in a signal transduction system that controls the expression of LH4 antenna complex genes (7,8). Each of these BphPs has a chromophore-binding domain and a C-terminal histidine kinase domain.…”

“…This finding is consistent with the conclusion that under semiaerobic conditions, BV is synthesized and incorporated into RpBphPs, allowing them to respond to red light. R. palustris also increases the synthesis of its LH4 complexes in response to a decrease in white light intensity when grown anaerobically, and both RpBphP2 and RpBphP3 are required for this response (7,8,16). In plants and cyanobacteria, phytochromes have been implicated in the sensing of light intensity, for which their ability to bind a chromophore and carry out photoconversion is essential (17,18).…”