The Intracellular Localization of Ribulose-1,5-Bisphosphate
Carboxylase/Oxygenase in Chlamydomonas
reinhardtii1
Olga N. Borkhsenious,
Catherine B. Mason, and
James V. Moroney*
Department of Biological Sciences, Louisiana State University,
Baton Rouge, Louisiana 70803
The pyrenoid is a proteinaceous
structure found in the chloroplast of most unicellular algae. Various
studies indicate that ribulose-1,5-bisphosphate carboxylase/oxygenase
(Rubisco) is present in the pyrenoid, although the fraction of Rubisco
localized there remains controversial. Estimates of the amount of
Rubisco in the pyrenoid of Chlamydomonas reinhardtii
range from 5% to nearly 100%. Using immunolocalization, the amount of
Rubisco localized to the pyrenoid or to the chloroplast stroma was
estimated for C. reinhardtii cells grown under different
conditions. It was observed that the amount of Rubisco in the pyrenoid
varied with growth condition; about 40% was in the pyrenoid when the
cells were grown under elevated CO2 and about 90% with
ambient CO2. In addition, it is likely that pyrenoidal
Rubisco is active in CO2 fixation because in vitro activity
measurements showed that most of the Rubisco must be active to account
for CO2-fixation rates observed in whole cells. These
results are consistent with the idea that the pyrenoid is the site of
CO2 fixation in C. reinhardtii and other
unicellular algae containing CO2-concentrating mechanisms.
1
This work was supported by the
National Science Foundation (grant no. IBN-9632087).
*
Corresponding author; e-mail btmoro{at}unix1.sncc.lsu.edu; fax
1-504-388-8459.
Plant Physiol. (1998) 116: 1585-1591
Copyright Clearance Center: 0032-0889/98/116/1585/07
© 1998 American Society of Plant Physiologists
