Mutagenesis studies on the phototropin-related protein YtvA from Bacillus subtilis have revealed the role of selected structural elements in interdomain communication. The LOV (light, oxygen, voltage) domain of YtvA undergoes light-driven reactions similar to that of phot-LOV, with reversible formation of a covalent flavin-cysteine adduct. The mutated proteins Ytva-E105L and YtvA-E56Q have been studied by UV fluorescence and circular dichroism (CD) spectroscopy. E105 (L in phototropin) is located at the solvent-exposed surface of the LOV domain central beta-sheet, demonstrated to participate in interdomain interaction in phototropin. CD data show that YtvA-E105L has a lower alpha-helix content in the dark and undergoes larger light-driven conformational changes than YtvA-WT. The E56Q mutation breaks the E56-K97 salt bridge, a structural element highly conserved within the LOV series. In YtvA-E56Q the CD spectrum is the same as in YtvA-WT, although the conserved W103 becomes more exposed to the solvent and the dark-recovery kinetics is slower. These results indicate that the E56-K97 salt bridge stabilizes locally the protein structure and participates in the regulation of the photocycle but has negligible effects on the overall structure. The E105L mutation, instead, highlights the involvement of the central beta-sheet in the light-driven conformational changes in LOV proteins.

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PACS. 42.50.Lc -Quantum fluctuations, quantum noise, and quantum jumps. PACS. 42.50.Ar -Photon statistics and coherence theory.Abstract. -Suppression of in-loop photocurrent fluctuation below the amplifier dark noise was observed in both a negative-feedback light-emitting diode (LED) and a laser diode (LD). We use a new semiclassical model to analyze the squeezed mechanism of the in-loop photocurrent noise mentioned above and find that there is a feedback-measurement-induced anticorrelation effect between the measured photocurrent fluctuation and the amplifier dark noise, which results in the suppression of the in-loop photocurrent fluctuation below the amplifier dark noise. Our theoretical analysis gives the correlation coefficient between the photocurrent fluctuation and amplifier noise, and also yields the squeezed spectrum of the in-loop intensity fluctuation according to our experimental results.c EDP Sciences