What They Saw
They grew Azotobacter vinelandii CA and purified its hydrogenase, then studied its subunits with mass spectrometry.
They observed that the actual size of the larger subunit was 64.9 kDa, smaller than the 66.6 predicted size. The N-terminal was still the same as predicted, so they concluded that about 15 amino acids had been removed from the C-terminal of the subunit. This appears to be necessary for it to function.