The closed complex is converted into an open complex of 1.3 turns of the double helix in a series of steps by first “melting” a short region of DNA around the -10 region, giving an unstable intermediate open complex within the sequence bound by the enzyme.  Two bases in the non-template strand of the –10 element (A11 and T7) flip out from their base-stacking interactions and instead insert into pockets within the s protein where they make more favorable interactions. By stabilizing the single-stranded form of the –10 element, these interactions drive melting of the promoter region.

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#Unit 6. System Physiology – Plant

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#Unit 6. System Physiology – Plant

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#Unit 6. System Physiology – Plant

Chaperonins GroEL and GroES sequester and ensure the correct folding of the newly synthesized large subunit (L). 

The assembly chaperone (RbcX)2 binds the large subunit and provides positional information for proper association of large subunit dimers to form the L8–((RbcX)2)8 complex. 

The small subunit (S) associates with L8–((RbcX)2)8, triggering a conformational change in RbcL and eliciting 

the release of (RbcX)2 and formation of the catalytically mature Rubisco holoenzyme [L]8–[S]8.

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#Unit 6. System Physiology – Plant

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#Unit 6. System Physiology – Plant

One way in which rubisco is activated involves the formation of a carbamate– Mg2+ complex on the ε-amino group of a lysine within the active site of the enzyme. Two protons are released. Activation is enhanced by the increase in Mg2+ concentration and higher pH (low H+ concentration) that result from illumination. The CO2 involved in the carbamate–Mg2+ reaction is not the same as the CO2 involved in the carboxylation of ribulose-1,5-bisphosphate.

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#Unit 6. System Physiology – Plant

The activity of rubisco is also regulated by light, but the enzyme itself does not respond to thioredoxin.

Two protons are released during the formation of the ternary complex rubisco–CO2–Mg2+, so activation is promoted by an increase in both pH and Mg2+ concentration.