Summary of all two component systems

Question 1

FixL/J

Answer 1

FixL/FixJ ensures nitrogenase is only expressed in the absence of O2 (nitrogenase is irreversibly inhibited by O2)

FixL = histidine kinase 
FixJ = response regulator (controls nitrogenase genes) 

O2 present: FixL off = no FixJ-P = no nitrogenase
O2 absent: FixL on = FixJ-P = nitrogenase

FixL/J system regulates the nifA and fixK operons which control expression of nitrogenase

FixL senses O2 by binding HAEM