Team:Cambridge/Bioluminescence/Background

The Lux System

Vibrio harveyi in the Bioglyphs project

The Lux operon is a set of genes active in bacterial luminescence. Homologues are found in different species of luminescent bacteria, such as Vibrio fischeri, Vibrio harveyi, Vibrio (formerly Photobacterium) phosphoreum, Photobacterium leiognathi and Photorhabdus (Xenorhabdus) luminescens. Between these species there are slight differences in the order of genes. In the most studied species, V. fischeri, the system consists of two translated regions, a leftward region containing the LuxR gene and a rightward region containing the genes LuxI, C, D, A, B, E and G in this order. LuxA and LuxB encode the two subunits of the bacterial luciferase, while the products of LuxC, LuxD and LuxE synthesize the substrate for the light emitting reaction, tetradecanal. The exact function of LuxG is unknown, and it appears to be non-essential for light emission, but its presence increases light output. Due to the specific codon usage in the Lux operon, LuxA and LuxB are translated at a five times higher level than C, D, E and G. In V.fischeri and many other species, the expression of the Lux system is tightly coupled with the intra-specific quorum sensing mechanisms. Thus bioluminescence is controlled by the physiological state of the cell (availability of substrates) as well as the state of the colony (concentration of quorum-sensing molecules)

Repression by H-NS

H-NS repression via (A) promoter occlusion and (B) Polymerase trapping

A nucleoid protein, H-NS, appears to be intricately involved in the regulation of the transcription of Lux genes. H-NS is a pleiotropic repressor protein that has been shown to bind predominantly to curved DNA caused by A-T rich regions. Repression can occur both by binding to promoter sites (promoter occlusion) and by looping the DNA around a promoter region to trap the RNA-Polymerase ([http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=eurekah&part=A31864 Pon et al. 2000]) The H-NS protein consist of a dimerization (or multimerization) and a DNA-binding domain. Its binding to DNA appears to be DNA shape- rather than sequence-specific. A description of the DNA binding properties of H-NS proteins can be found in ([http://www.nature.com/nature/journal/v444/n7117/full/nature05283.html Dame et al. 2006]) The protein has also been implicated in the repression of foreign genes acquired by horizontal transfer and synthetic genes ([http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6976.2008.00155.x/pdf Dorman and Kane 2008]). Certain h-ns mutants have been shown to exhibit much higher levels of gene expression and bioluminescence. Several sites within the Lux system have been described as binding sites for H-NS ([http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1099-1271(199807/08)13:4%3C185::AID-BIO486%3E3.0.CO;2-U/abstract Ulitzur 1998]). Such binding sites exist in both the leftward and the rightward promoter regions, within the coding sequence of LuxI, the intergenic region and start of LuxC as well as the intergenic region and start of LuxA.

Regulation by LuxR and LuxI

transcriptional induction of gene expression by LuxR+AHL