BLUE LIGHT PHOTORECEPTORS FROM PLANT SYMBIOTIC BACTERIA

In recent years novel and largely unforeseen biological photoreceptors have been discovered in many bacteria, in most cases with poorly understood in vivo functions. Bacterial photoreceptors mainly belong to two superfamilies: blue light (BL) sensing LOV proteins and red/far red (R/FR) light sensing bacteriophytochromes (BphP), binding respectively flavin mononucleotide (FMN) and biliverdin-IXa (BV) as chromophores. LOV proteins and phytochromes are also the main photoreceptors of plants, and it is clear that many bacteria that are plant pathogens or symbionts are able to detect the same colors as their natural host. In this paper we will present the biophysical characterization of novel BL receptors from Methylobacterium radiotolerans, a radiation resistant, nitrogen fixing bacterium, able to promote plant growth and grow facultatively on methanol (2). In addition, M. radiotolerans is an opportunistic human pathogen and has a high potential for being employed in soil bioremediation. As other Methylobacteria, M. radiotolerans bears genes for several BphP and BL receptors, that we started to investigate during the last months by means of steady state and time-resolved spectroscopy. In particular we focus here on a LOV protein that show high structural stability and an extremely long photocycle in its wild type form. Sequence analysis revealed some peculiarities with respect to the majority of LOV domains; point mutations evidenced that this M. radiotolerans LOV photoreceptor is a promising candidate for biophysical applications, chiefly as fluorescent reporter and as genetically encoded photosensitizer (3). The possible roles of photoreceptors in the physiology of M. radiotolerans is also discussed, on the basis of bioinformatics analysis.