Noncanonical Photodynamics of the Orange/Green Cyanobacteriochrome Power Sensor NpF2164g7 from the PtxD Phototaxis Regulator of Nostoc punctiforme

Abstract

Forward and reverse primary (<10 ns) and secondary (>10 ns) photodynamics of cyanobacteriochrome (CBCR) NpF2164g7 were characterized by global analysis of ultrafast broadband transient absorption measurements. NpF2164g7 is the most C-terminal bilin-binding GAF domain in the Nostoc punctiforme phototaxis sensor PtxD (locus Npun_F2164). Although a member of the canonical red/green CBCR subfamily phylogenetically, NpF2164g7 exhibits an orange-absorbing P dark-adapted state instead of the typical red-absorbing P dark-adapted state characteristic of this subfamily. The green-absorbing P photoproduct of NpF2164g7 is unstable, allowing this CBCR domain to function as a power sensor. Photoexcitation of the P state triggers inhomogeneous excited-state dynamics with three spectrally and temporally distinguishable pathways to generate the light-adapted P state in high yield (estimated at 25-30%). Although observed in other CBCR domains, the inhomogeneity in NpF2164g7 extends far into secondary relaxation dynamics (10 ns -1 ms) through to formation of P. In the reverse direction, the primary dynamics after photoexcitation of P are qualitatively similar to those of other red/green CBCRs, but secondary dynamics involve a "pre-equilibrium" step before regenerating P. The anomalous photodynamics of NpF2164g7 may reflect an evolutionary adaptation of CBCR sensors that function as broadband light intensity sensors.

Conservation and Diversity in the Primary Reverse Photodynamics of the Canonical Red/Green Cyanobacteriochrome Family

“…As observed with other studies of CBCR ultrafast dynamics, ,, not all photogenerated 15 E P g * populations yielded a primary Lumi-G o photointermediate. The SADS of photoactive 15 E P g * subpopulations typically share similar spectral properties, whereas the non-photoactive 15 E P g * populations often exhibit altered SADS features (Figure C and Figure S10).…”

“…The observation of unproductive 15 E P g * subpopulations in one or more CBCRs in all three classes suggests that inhomogeneity is a consistent trend for the reverse photoreaction of all three classes of red/green CBCRs. Such unproductive excited-state populations likely are present in the majority of photoactive biliproteins, as they have been reported in noncanonical red/green (NpR3784), noncanonical orange/green (NpF2164g7), green/red (RcaE), , and insert-cysteine (NpF2164g3) CBCRs. For the reverse reactions of NpF2164g3 and NpF2164g7 as well as both forward and reverse reactions of RcaE, , the spectra of photoinactive and photoactive excited-state populations were indistinguishable.…”

The cyanobacterial taxis protein HmpF regulates type IV pilus activity in response to light

“…The Hmp system senses light indirectly, possibly via alterations in PMF, and is probably responsive only to drastic increases, and perhaps decreases, in light intensity, whereas the Ptx system senses light more directly through its GAF domains and is responsive to more subtle changes in light intensity. This is consistent with the role of the Ptx system in macroscopic colonyspreading assays where filaments are exposed to a gradation in light intensity and also with the finding that the MCP of the Ptx system is a broad-spectrum power sensor (33). It is unclear at this point if the Ptx system also modulates T4P activity via HmpF or through another independent means of regulation.…”

Comparison of the Forward and Reverse Photocycle Dynamics of Two Highly Similar Canonical Red/Green Cyanobacteriochromes Reveals Unexpected Differences

“…A unique feature of the reverse dynamics of AnPixJg2 is the presence of the Meta-G f intermediate red-shifted from the 15 Z P r photoproduct (Figure B). Photointermediates red-shifted from the photoproduct absorption have been observed at later stages of the reverse reaction for NpR5113g2 (red/green), NpF2164g7 (orange/green; B2J668), and Slr1393 (P73184). The presence of comparable intermediate populations in the reverse dynamics of canonical red/green CBCRs and a noncanonical orange/green CBCR suggests that this population may be more ubiquitous in the reverse pathway of CBCRs.…”

“…Canonical red/green CBCRs have been extensively studied by ultrafast (femtoseconds to nanoseconds) − and long time (nanoseconds to milliseconds) ,,− spectroscopies. However, the molecular basis for differences in red/green CBCR excited-state lifetimes and time scales of the primary Lumi photointermediate formation in the forward direction is unresolved.…”