Molecular Genetics of Photosynthesis and Carbon Assimilation in Plants 
chaperonins (Cpn60) a and ^ have been sug- 
gested as participating in RuBisCO assembly. Since 
the availability of functional RuBisCO is important 
for carbon assimilation, we have isolated two 
cDNAs and a genomic clone for Cpn60 /?. 
To study whether Cpn60 ^ is indeed involved 
in RuBisCO assembly, and whether it is specific 
for'RuBisCO or plays a more general role in pro- 
tein assembly, we are currently generating trans- 
genic plants containing a chimeric gene to pro- 
duce an antisense RNA for Cpn60 /3. The RNA 
should interact with the Cpn60 /3 mRNA and 
arrest the production of Cpn60 /3 polypeptides, 
thus producing transgenic plants that will allow 
us to assess the role of Cpn60 (8 in RuBisCO assem- 
bly. We are also constructing chimeric genes com- 
posed of the promoter of a Cpn60 ^ gene and the 
coding sequence of the /^-glucuronidase bacterial 
reporter gene, to study the tissue-specific and en- 
vironmental regulation of this gene in transgenic 
plants. 
Again, the main assimilates from photosyn- 
thesis are triose phosphate molecules that are 
converted into sucrose to be translocated to con- 
sumer tissues. Several physiological and biochem- 
ical studies indicate that sucrose 6-phosphate syn- 
thase (SPS) is the limiting enzyme for this 
conversion. To study the role of SPS in carbon 
assimilation, and the interrelation of the genes 
encoding this enzyme with photosynthetic genes, 
we are currently isolating SPS cDNA clones, using 
monospecific SPS antibodies and polymerase 
chain reaction technology in collaboration with 
Horacio Pontis from Mar del Plata-Argentina. 
(The work on SPS is supported in part by a grant 
from the Rockefeller Foundation.) 
Opposite: Light microscopy of transverse stem 
sections of transgenic tobacco plants harbor- 
ing a chimeric gene in which the P-glucuroni- 
dase-coding sequence is under control of the 
chaperonin 60 0 promoter. The blue staining 
indicates the cell- type- specific expression di- 
rected by the promoter at the basal (A) and 
apical (B) stem regions of transgenic plants. 
Research and photograph by Eduardo Zaba- 
leta in the laboratory of Luis Herrera-Estrella. 
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