|Content of Engineering Photosynthetic Pathways
RuBisCO is the rate-limiting enzyme in plant photosynthesis. Under the present
model for photosynthesis, it should be possible to increase CO2 fixation in C3 plants by about 20%, before entering RuBP- and Pi-limited phases (Sage, 1990;
Sage et al., 1989). Since the PCR cycle is the major consumer of energy formed at the thylakoids (Heldt, 1997), alterations of the enzyme should guarantee that the
PCR cycle would siphon off and productively utilize more energy with an
improved enzyme. Several directions about how to accomplish such improvement
have been discussed (Andrews and Whitney, 2003; Parry et al., 2003).
However, another strategy would be to engineer a RuBisCO enzyme that
continued to fix CO2 under drought conditions when stomata aperture is reduced.
First, we need to know which partial reaction of the enzyme constitutes the
limiting step and which residues might determine the enzymatic properties
(Mauser et al., 2001). Second, based on the detection of naturally occurring
RuBisCO enzymes that are superior to the plant enzyme, work may be directed
to replace resident rbcL (and rbcS) gene in plastid and nuclear DNA with the genes
coding for the superior enzyme (Andrews and Whitney, 2003; Parry et al., 2003).
Integration of the information from research with these superior enzymes
suggests the possibility to engineer a higher plant rbcL gene that incorporates
sequences responsible for improved RuBisCO performance. However, incorporating
such engineered chimeric genes into chloroplast DNA faces challenges and
obstacles that need to be addressed.