Light-sensing in bacteria

Abstract

Light reception plays an important role in regulating lifestyle changes in bacteria. My research focused on a number of different light-sensing systems in different bacteria: PpaA in Rhodobacter sphaeroides, PixD-PixE in Synechocystis sp. PCC, and the known light-receptors in Rhodospirillum centenum.

PpaA from R. sphaeroides had previously been shown to be a heme-binding protein, despite its sequence similarity to cobalamin-binding proteins. My research showed that PpaA is in fact a bona fide cobalamin-binding protein. PpaA binds specifically hydroxy-cobalamin, but not other forms of cobalamin. PpaA does have some ability to bind heme, but a mutant form of PpaA that showed better heme-binding was inactive in vivo. This suggests that PpaA functional cofactor is cobalamin, rather than heme.

We also tested cobalamin-binding in a number of homologs of PpaA and found that almost all are indeed cobalamin-binding proteins.

The genome of Rhodospirillum centenum contains four reading frames that encode light sensing proteins. We identified possible role for each of these light-receptors by making deletion mutants, and testing the impact of these deletion on the transcription levels. Our results suggest that the PYP-phytochrome hybrid Ppr plays a role as a global regulator of transcription. A BLUF and a bacteriophytochrome on the other hand showed changes in expression levels of a number of genes involved in motility. Lastly, deletion of a LOV domain containing protein did not result in any significant changes in gene expression levels. This suggests that the LOV protein does not regulate life-style changes, but rather controls immediate responses.

PixD is a short BLUF protein that has been shown to play a role in regulating phototaxis in Synechocystis sp. PCC6803. We were able to show that Slr1692, encoded by an ORF upstream of pixE-pixD might play a role in regulating phototaxis.