Locked Chromophore Analogs Reveal That Photoactive Yellow Protein Regulates Biofilm Formation in the Deep Sea Bacterium <i>Idiomarina loihiensis</i> Michael A. van der Horst T. Page Stalcup Sandip Kaledhonkar Masato Kumauchi Miwa Hara Aihua Xie Klaas J. Hellingwerf Wouter D. Hoff 10.1021/ja9057103.s001 https://acs.figshare.com/articles/journal_contribution/Locked_Chromophore_Analogs_Reveal_That_Photoactive_Yellow_Protein_Regulates_Biofilm_Formation_in_the_Deep_Sea_Bacterium_i_Idiomarina_loihiensis_i_/2809378 <i>Idiomarina loihiensis</i> is a heterotrophic deep sea bacterium with no known photobiology. We show that light suppresses biofilm formation in this organism. The genome of <i>I. loihiensis</i> encodes a single photoreceptor protein: a homologue of photoactive yellow protein (PYP), a blue light receptor with photochemistry based on trans to cis isomerization of its <i>p</i>-coumaric acid (<i>p</i>CA) chromophore. The addition of trans-locked <i>p</i>CA to <i>I. loihiensis</i> increases biofilm formation, whereas cis-locked <i>p</i>CA decreases it. This demonstrates that the PYP homologue regulates biofilm formation in <i>I. loihiensis</i>, revealing an unexpected functional versatility in the PYP family of photoreceptors. These results imply that <i>I. loihiensis</i> thrives not only in the deep sea but also near the water surface and provide an example of genome-based discovery of photophysiological responses. The use of locked <i>p</i>CA analogs is a novel and generally applicable pharmacochemical tool to study the in vivo role of PYPs irrespective of genetic accessibility. Heterologously produced PYP from <i>I. loihiensis</i> (Il PYP) absorbs maximally at 446 nm and has a <i>p</i>CA p<i>K</i><sub>a</sub> of 3.4. Photoexcitation triggers the formation of a pB signaling state that decays with a time constant of 0.3 s. FTIR difference signals at 1726 and 1497 cm<sup>−1</sup> reveal that active-site proton transfer during the photocycle is conserved in Il PYP. It has been proposed that a correlation exists between the lifetime of a photoreceptor signaling state and the time scale of the biological response that it regulates. The data presented here provide an example of a protein with a rapid photocycle that regulates a slow biological response. 2009-12-02 00:00:00 PYP biofilm formation FTIR difference signals Chromophore Analogs Reveal 3.4. Photoexcitation triggers loihiensis increases biofilm formation photoreceptor Deep Sea Bacterium Idiomarina loihiensisIdiomarina loihiensis protein pCA Photoactive Yellow Protein Regulates Biofilm Formation response