A good expression vector for protein expression in coli should have features like an origin of replication, a selectable marker, a promotor, an operator, and a ribosome binding site.
A Ribosome Binding Site (RBS) is a part or segment of the upstream part of a messenger RNA (mRNA) molecule which binds the ribosome to position the message correctly for the initiation of translation.
Selectable marker genes are conditionally dominant genes that confer an ability to grow in the presence of applied selective agents that are normally toxic to cells or inhibitory to growth, such as antibiotics and herbicides.
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I'm assuming this question refers to the rate of photosynthesis.
Plants need both light and CO2 to photosynthesize (PHOTOsynthesis means using light to use CO2 to make O2 and other nutrients). If the light intensity were very low, the plant wouldn't have enough energy to photosynthesize much.
You could have all the CO2 in the world, but if there's not enough light, the plant wouldn't be able to photosynthesize.
In short, it'd be difficult to find the ideal CO2 level if the light intensity were very low because the plant needs a certain light level to even photosynthesize. You could only find the ideal CO2 level if you had enough of all other nutrients, and CO2 level was the only variable you were changing.