INDUCTION OF THE CO_2-CONCENTRATING MECHANISM IS LIGHT DEPENDENT IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS

摘要

Under CO_2 limitation, active, inward transport of CO_2 and HCO_3~-supports accumulation of HCO_3~- in the cyanobacterial cytosol, which diffuses into the carboxysome where it is catalytically equilibrated by carbonic anhydrase ensuring an elevated supply of CO_2 to a kinetically inefficient Rubisco. This so-called CO_2-coneentrating mechanism (CCM) has been detected in many species of cyanobacteria and microalgae and is an efficient adaptation to CO_2 limitation in natural settings (Price et al 1998, Kaplan & Reinhold 1999). Under Ci limitation, the cellular affinity for Ci is enhanced as a result of de novo synthesis of several high affinity CO_2 and HCO_3~-- transporters, along with other components essential for CCM function. How the cell sensesCi limitation and transmits the primary signals to the transcriptional machinery is, however, not known. Candidate signals include either direct sensing of external and/or internal Ci pools, the redox state of the photosynthetic electron transport chainor possibly a key metabolic intermediate in the photorespiratory cycle. In many studies of CCM regulation, it has largely been assumed that some sort of light signal is required for induction, although often little attention is paid to the intensity orspectral quality of light applied to growing cultures or experimental cell suspensions. We tested whether light regulates induction of the CCM in Syneehococcus elongatus by using quantitative RT-PCR to monitor the abundance of key low-Ci inducible transcripts under variable experimental conditions.