دانلود رایگان مقاله خصوصیات فتوشیمیایی رودوپسین اکتین و وجود عملکردی آن در میزبان طبیعی

ABSTRACT

Actinorhodopsin (ActR) is a light-driven outward H+ pump. Although the genes of ActRs are widely spread among freshwater bacterioplankton, there are no prior data on their functional expression in native cell membranes. Here, we demonstrate ActR phototrophy in the native actinobacterium. Genome analysis showed that Candidatus Rhodoluna planktonica, a freshwater actinobacterium, encodes one microbial rhodopsin (RpActR) belonging to the ActR family. Reflecting the functional expression of RpActR, illumination induced the acidification of the actinobacterial cell suspension and then elevated the ATP content inside the cells. The photochemistry of RpActR was also examined using heterologously expressed RpActR in Escherichia coli membranes. The purified RpActR showed λmax at 534 nm and underwent a photocycle characterized by the very fast formation of M intermediate. The subsequent intermediate, named P620, could be assigned to the O intermediate in other H+ pumps. In contrast to conventional O, the accumulation of P620 remains prominent, even at high pH. Flash-induced absorbance changes suggested that there exists only one kind of photocycle at any pH. However, above pH 7, RpActR shows heterogeneity in the H+ transfer sequences: one first captures H+ and then releases it during the formation and decay of P620, while the other first releases H+ prior to H+ uptake during P620 formation.

4. Conclusions

In this study, we present the first observation of ActR phototrophy in native actinobacterial cells. Without the help of exogenous retinal, RpActR showed light-induced H+ pump activity and drove ATP production in its natural host cells. This result confirms the predicted signifi- cance of ActRs to the solar-energy inflow into freshwater ecosystems. Using the purified RpActR, we found several interesting features in the photocycle. M formation is extremely fast: It is competed before 10 μs and could not be followed by our apparatus. Compared to other H+ pumps, there appears to be no specific difference in the residues surrounding both the PSB and the H+ acceptor Asp92 (Fig. S1). Thus, the fast H+ transfer between them might be caused by their specific coordination in the photolyzed state. After M formation, P620 and RpActR′ appear sequentially. P620 could be assigned to O in BR and other H+ pumps. However, unlike conventional O, the decay of P620 shows pHdependent retardation. This decay is probably governed by the protonation state of a certain residue. At pH 6, RpActR first captures H+ during P620 formation and then releases H+ during P620 decay. Above pH 7, approximately half of RpActR releases H+ prior to uptake during P620 formation. The natural host of RpActR inhabits a pH-neutral freshwater pond. Thus, in the natural environment, RpActR probably shows heterogeneity in its H+ transfer sequences. Does this heterogeneity confer a survival advantage to the cells in the freshwater environment? This question is an interesting topic for future investigation.