| Thylakoid membrane biogenesis was studied in the cyanobacterium Synechocystis sp. strain PCC 6803 (hereafter referred to as Synechocystis) that is thought to be related to the endosymbionts that gave rise to chloroplasts. To elucidate primary elements in thylakoid membrane biogenesis, the study consisted of two approaches: (1) Disruption and overexpression of the vipp1 gene that is thought to be directly involved in thylakoid membrane synthesis. (2) Induction of thylakoid membrane synthesis by an environmental stimulus using a Synechocystis strain that is disrupted in its light-independent protochiorophyllide (Pchlide) reduction (chilL- Synechocystis ); when cultivated in darkness, this provides a system analogous to etiolated plastids. Disruption of vipp1 resulted in a merodiploid strain that retained thylakoids, implicating a crucial function of Vipp1 for cell viability. The Synechocystis strain overexpressing vipp1 (Highvipp) had enhanced lipid content, increased thylakoid membrane abundance, and altered stoichiometry of the photosystems. The Highvipp strain showed an altered lipid content: a doubling of the anionic lipid, sulfoquinovosyl diacylglycerol (SQDG) and increased polyunsaturation of the other lipid classes. Vipp1 was localized near the cytoplasmic membrane and was shown to associate with the ClpB2 protein, a chaperone belonging to the HSP100 family. The Synechocystis chilL - strain grown under LAHG (light-activated heterotrophic growth: dark except for one 15-minute light period per day) conditions accumulated Pchlide, showed severe reduction of thylakoid membranes, and contained multiple cytoplasmic inclusions with one population distinct by its enclosure by a membrane. Upon brief illumination of the LAHG-grown chilL- strain, thylakoid membranes were synthesized from the membrane-enclosed inclusions. The kinetics of thylakoid membrane synthesis during illumination of the LAHG-grown chlL strain did not correlate with photosynthetic competence, which was achieved at a later stage. The results show that Vipp1 has a primary role in thylakoid membrane biogenesis whereas chlorophyll availability may be a trigger of membrane synthesis but lags thylakoid membrane accumulation. It is proposed that: (1) Vipp1 and SQDG are important elements in a vesicular membrane transport machinery that may be analogous to the eukaryotic one; and (2) Synechocystis possesses a prolamellar-body-like structure that plays an important role in thylakoid membrane biosynthesis and maintenance. |
