Regulation of photosynthesis in the unicellular, diazatrophic cyanobacterium Cyanothece sp. ATCC 51142

Don Lee Tucker, Purdue University

Abstract

Cyanothece sp. Strain ATCC 51142 is a unicellular, aerobic, diazotrophic cyanobacterium that maintains a temporal separation of N 2 fixation and photosynthetic O2 evolution during its diurnal cycle in order to protect the O2-labile nitrogenase enzyme. We were interested in the mechanisms for regulating photosystem II and O 2 evolution during the Cyanothece sp. diurnal cycle. A stable in vitro system for studying Cyanothece sp. PSI and PSII reaction centers was developed which revealed that PSII O2 evolution regulation was maintained in O2-evolving extracts. PSI activity also cycled in the in vitro extracts, with a peak of activity prior to N2 fixation. Analysis of the Manganese Stabilizing Protein (MSP) was performed to determine its involvement in O 2 evolution regulation. Substantial regulation of psbO transcription (codes for MSP) was identified in Cyanothece sp., but the MSP concentration did not appear to vary during the diurnal cycle. These results, along with the larger apparent mass of Cyanothece sp. MSP (33kDa, compared to 29kDa in other cyanobacteria), implicated a possible regulatory region in Cyanothece sp. MSP. The N-terminal 100 amino acids of MSP from cyanobacteria were somewhat distinct from this region in plants, and secondary structure analysis also revealed some differences between plant and cyanobacterial MSP structures. Photosystem oligomerization may be involved in PSI and PSII regulation, and lead to the so-called state transitions. Non-denaturing green gels were employed to separate the major Chl-protein complexes in Cyanothece sp. The majority of the Chl-protein complexes did not vary during the diurnal cycle, but a single high molecular mass complex appeared transiently during the late light period and appeared to be composed of PSII dimers. The appearance of this complex coincides with the highest O2 evolution during the diurnal cycle indicating that PSH oligomerization may play a role in O2 evolution regulation in Cyanothece sp. Protein phosphorylation could play a role in regulating PSI and PSII activities. In vitro and in vivo phosphorylation of proteins was performed in Cyanothece sp. Some of these proteins were membrane associated and appeared to be phosphorylated in response to light or cellular redox state, indicating a possible role in photosynthetic regulation.

Degree

Ph.D.

Advisors

Sherman, Purdue University.

Subject Area

Molecular biology

COinS