Physiological Funcation Analysis Of Cyanobacterial NdhM Subunit

About20years ago, a NADPH dehydrogenase (NDH-1) complex was identified incyanobacteria and essential to cyclic electron transport around photosystem I (CET),respiration, and CO2uptake. Recently, many considerable achievements ofcyanobacterial NDH-1complex have been obtained, for example, a novel NdhSsubunit was identified in a unicellular cyanobacterium Synechocysitis sp. strain PCC6803(hereafter referred to Synechocystis6803). This subunit was assumed to locatein the OPS (Oxygenic Photosyntheisis-Specific) domain of NDH-1complex andinvolved in CET. However, except for NdhL and NdhS, little is known regarding thephysiological roles of other OPS domain subunits, such as NdhM, N and O.In cyanobacteria, CET mainly consists of NDH-1-mediated one (NDH-CET) andPGR5-mediated one (PGR5-CET). To screen the genes related to the activity ofNDH-CET, this thesis first constructed and obtained the Δpgr5mutant that lacks theactivity of PGR5-CET. Secondly, we obtained two high light-sensitive mutants byscreening a transposon-tagged library of Synechocystis6803in thePGR5-CET-defective background. Further, the results indicated that such highlight-sensitive growth phyenotype was casued by the impairment of NDH-CET, andthe transposon insetin occurred in the slr1623gene that encodes a known protein,NdhM subunit. Thirdly, to clarify the physiological function of NdhM, we constructedand obtained the ndhM deletion single mutant (ΔndhM) and ndhM/pgr5doublemutant (ΔndhM/pgr5), respectively. Further, the results indicated that mutation ofNdhM subunit did not affect the accumulation, assembly and activity of NDH-1L and NDH-1M complexes. However, mutation of NdhM subunit remarkably decreased therate of CET under the PGR5-CET-defective background, although the accumulationand assembly of NDH-1complexes were still unaffected. Finally, this thesis foundthat NdhM protein interacted with NdhK subunit by using the yeast two-hybridsystem.Based on the aforementioned results, we proposed that mutation of NdhM mayinfluence the rate of electron transfer in NdhK protein, thereby imparing the activityof NDH-CET, which was mostly complemented by the PGR5-CET activity. However,the impaired NDH-CET activity can not be complemented under thePGR5-CET-defective background, and thus producing the high light-sensitivephenotype of growth.