LIC-SEMINAR

Kara Barker-Åström
Department of Plant Physiology
Umeå university

"The Role of ClpP Protease in Phycobilisome Degradation During Nutrient Stress in Synechococcus elongatus"

Time: Thursday October 14, 10.00
Place: KB3A9, KBC

Host: Petter Gustafsson

ABSTRACT

 

Free-living photosynthetic organisms must cope with wide fluctuations in environmental conditions such as light, temperature or nutrient availability.  In cyanobacteria, degradation of the light-harvesting complexes, the phycobilisomes, is an adapative response that enables them to withstand long periods of macronutrient deprivation.  In this Licensiate thesis, I have examined this process more thoroughly, specifically focusing on the role of Clp protease in nutrient starvation-induced phycobilisome degradation in the cyanobacterium Synechococcus elongatus (formerly Synechococcus sp. PCC ). 

We have shown that the ClpP1 subunit of the Clp protease is involved in pigment degradation during nutrient stress.  Phycocyanin degradation, a sign of phycobilisome loss, occurs during sulfur starvation in wild-type cells but was blocked in a clpPI inactivation mutant.  The ClpP1 component of the Clp protease was detected in cross-linked phycobilisome samples isolated from cultures grown in sulfur-deplete media, but not from cultures grown in sulfur-replete media.  Under nitrogen deficiency we found that ClpP1 is necessary for chlorophyll degradation.  Phycocyanin degradation was not dependent on an intact clpPI gene during nitrogen deprivation, however, indicating that the pigment-degrading activity of the ClpP1 proteolytic subunit differs depending on environmental conditions.  This might be a result of the association between the ClpP1 proteolytic component of the Clp protease and the ATPase regulatory component, which confers substrate specificity to the Clp protease.  Two potential Clp protease ATPase regulatory partners, ClpC and GroEL, were detected in phycobilisomes isolated from sulfur-deprived S. elongatus cultures.  ClpC was absent and GroEL was present only in very low amounts in phycobilisomes isolated from control cultures grown in sulfur-replete media.  Furthermore, we have demonstrated that bleaching during sulfur and nitrogen starvation in wild-type cells grown under the same conditions can differ markedly.  When cultures of S. elongatus were grown at 37°C, with 120 mmol photons m^-2 s^-1, and 5% CO₂ and then transferred to sulfur-deficient media, phycocyanin was degraded although chlorophyll content remained constant on a culture basis but decreased on a per cell basis by dilution over a round of cell division after pigment accumulation had ceased.  In contrast, when the cultures were grown under the same conditions but transferred to nitrogen-deficient media, chlorophyll was rapidly degraded and phycocyanin content per cell decreased primarily by dilution.  By systematically altering growth parameters, we were able to demonstrate that pigment losses during nitrogen deficiency depend on growth temperature and carbon dioxide status.