Abstract
Light is an environmental signal that produces extensive effects on the physiology of the human pathogen Acinetobacter baumannii. Many of the bacterial responses to light depend on BlsA, a bluelight using FAD (BLUF)-type photoreceptor, which also integrates temperature signals. In this work, we disclose novel mechanistic aspects of the function of BlsA. First, we show that light modulation of motility occurs only at temperatures lower than 24°C, a phenotype depending on BlsA. Second, blsA transcript levels were significantly reduced at temperatures higher than 25°C, in agreement with BlsA protein levels in the cell which were undetectable at 26°C and higher temperatures. Also, quantum yield of photo-activation of BlsA (lBlsA) between 14 and 37°C, showed that BlsA photoactivity is greatly compromised at 25°C and absent above 28°C. Fluorescence emission and anisotropy of the cofactor together with the intrinsic protein fluorescence studies suggest that the FAD binding site is more susceptible to structural changes caused by increments in temperature than other regions of the protein. Moreover, BlsA itself gains structural instability and strongly aggregates at temperatures above 30°C. Overall, BlsA is a low to moderate temperature photoreceptor, whose functioning is highly regulated in the cell, with control points at expression of the cognate gene as well as photoactivity.
Highlights
Blue light modulates multiple cellular processes in the important human pathogen Acinetobacter baumannii including metabolic pathways such as the phenylacetic acid (PAA) degradation pathway and trehalose biosynthesis, tolerance to some antibiotics as well as antioxidant enzyme levels such as catalase
It can be observed that bacterial motility was only inhibited between 18 and 24◦C under blue light illumination, while photoregulation was lost above 24◦C and the bacteria moved throughout the plates at this condition
It has been recently reported that motility depends on the pilus assembly system PrpABCD at 24◦C, with differential expression of prpA in response to light in a BlsA-dependent manner in A. baumannii ATCC 17978 (Wood et al, 2018), confirming our previous results showing dependence on BlsA of this phenotype (Mussi et al, 2010)
Summary
Blue light modulates multiple cellular processes in the important human pathogen Acinetobacter baumannii including metabolic pathways such as the phenylacetic acid (PAA) degradation pathway and trehalose biosynthesis, tolerance to some antibiotics as well as antioxidant enzyme levels such as catalase. These traits likely contribute to the bacterial persistence in adverse environments (Müller et al, 2017). BlsA Is a Low to Moderate Temperature Photoreceptor previous work, we further showed that in A. baumannii, blue light modulates motility, biofilm formation, and virulence against Candida albicans (Mussi et al, 2010) Most of these blue light-depending processes are governed by BlsA, a Blue Light Using FAD (BLUF) photoreceptor. BlsA is the only so far shown to function as a global regulator
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