Molecular Genetics of Photosynthesis 
and Carbon Assimilation in Plants 
Luis R. Herrera-Estrella, Ph.D. — International Research Scholar 
Dr. Herrera-Estrella is Professor and Head of the Department of Plant Genetic Engineering at the Center 
for Research and Advanced Studies, National Polytechnic Institute, Irapuato. He received his 
undergraduate degree as biochemical engineer from the National Polytechnic Institute in Mexico and his 
Ph.D. degree from the State University of Ghent, Belgium, where his thesis advisors were Marc Van 
Montagu and Jeff Schell. The subject of his thesis was the expression of foreign genes in plant cells. As a 
postdoctoral researcher in Ghent, he studied the regulation of light- inducible plant genes and the 
movement of proteins into the chloroplast. His honors include the Minoru and Ethel Tsutsui Distinguished 
Graduate Research Award in Science from the New York Academy of Sciences and the Javed Husain Award 
for Young Scientists from UNESCO. 
PHOTOSYNTHESIS and carbon assimilation 
are the most important biochemical and mo- 
lecular events in the life cycle of higher plants 
and, indeed, are key to the provision of nutrients 
for the entire food chain. Solar energy is first col- 
lected in the chloroplasts of photosynthetic tis- 
sues, mainly of leaves, by light-harvesting anten- 
nas composed of chlorophyll and protein 
molecules. The collected energy is then used to 
convert atmospheric carbon dioxide (CO2) into 
triose phosphate molecules. These three-carbon 
molecules proceed through a series of reactions, 
called the Calvin-Benson cycle, that culminates 
in the production of sugars from v^^hich all the 
organic molecules required for plant life are 
synthesized. 
More specifically, triose phosphate molecules 
are converted in the cytoplasm of photosynthe- 
tic, or source, cells into sucrose, which is translo- 
cated through the phloem to feed the nonphoto- 
synthetic, or consumer, tissues (i.e., roots, 
flowers, seeds, tubers). Assimilated carbon is 
stored temporarily or permanently in the form of 
starch in both source and consumer tissues. The 
starch in seeds or tubers provides most of the car- 
bon and energy for the germination and develop- 
ment of new plants. 
The light-dependent production of ATP and 
NADPH, the reductive assimilation of CO2, and 
sucrose and starch synthesis are interlinked and 
interdependent. These processes must be coordi- 
nated in vivo at both the biochemical and genetic 
level (i.e., at the level of gene expression). The 
balance between the efficiency of CO2 fixation, 
sucrose translocation and uptake, and assimila- 
tion of sucrose in consumer tissues plays a funda- 
mental role in determining the productivity of 
any given plant species. This balance is affected 
by both genetic determinants of the individual 
and its interaction with the environment. 
Our laboratory is interested in studying the mo- 
lecular events that control the biochemical pro- 
cesses involved in carbon assimilation in plant 
cells. One aspect that we are investigating is how 
light regulates genes involved in photosynthesis. 
Ribulose 1 ,5-bisphosphate carboxylase oxygen- 
ase (RuBisCO) is a multimeric enzyme composed 
of eight identical small subunits (SS) and eight 
identical large subunits (LS) . In the so-called C3 
plants, RuBisCO carries out the initial CO2 fixa- 
tion step, utilizing the five-carbon sugar ribulose 
bisphosphate to produce two three-carbon deriva- 
tives (triose phosphate molecules). The gene 
family encoding the SS is located in the nuclear 
genome; the gene encoding the LS is located in 
the plastid genome. How genes located in differ- 
ent cellular compartments are coordinately regu- 
lated (those for both SS and LS are regulated by 
light and should produce the corresponding sub- 
units in equimolar amounts) , and how the active 
RuBisCO enzyme is assembled from its subunits, 
are some of the questions we wish to answer. 
In collaboration with June Simpson, we 
showed previously that the 5'-flanking sequences 
of genes encoding the small subunit of the RuBis- 
CO {ss genes) and the chlorophyll «/fo-binding 
proteins (cabSO gene) are responsible for the 
light-inducible transcription of these genes. We 
have also shown that enhancer and silencer ele- 
ments are involved in the tissue-specific and 
light-inducible expression of the cabSO gene. 
The cis- and trans-acting elements involved in the 
regulation of the cabSO gene are being analyzed. 
From these studies a 7-base pair repeated ele- 
ment within the 5'-flanking region of the cabSO 
gene has been identified as the site of interaction 
with a putative regulatory DNA-binding protein. 
Many attempts to assemble RuBisCO in vitro 
from its isolated subunits have failed, suggesting 
that the plant cell in vivo provides other req- 
uisite components. It has been suggested that 
RuBisCO assembly requires the participation of 
proteins that act as molecular chaperones to pro- 
mote the correct interaction between the sub- 
units, ensuring the assembly of the catalytically 
active enzyme. Two 60-kDa proteins termed 
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