Connecting B to A

Looking back on the bioinformatic analysis of eukaryotic nitrile hydratases in Marron, Akam and Walker's PLOS One paper to be found here, I thought it would be interesting to look at the link between the two segments which are separate subunits in prokaryotic NHases.
I mocked up this model of where the link had to go from two prokaryotic subunits which had most similarity to the Monosiga brevicollis NHase. The dangling ends of the right hand side of the structure indicate where the linker has to go.

As the easiest to access, I chose to align the protein sequences suggested by the transcriptions of ESTs from the four organisms held within the Broad Institute's Origins of Multicellularity project, namely M. brevicollis, Thecamonas trahens, Salpingoeca rosetta and Sphaeroforma arctica. (NB order in alignment: MB, SR, TT and last SA)

As Marron's paper comments:
"This histidine-rich region is prominent in T. trahens (12 residues), S. arctica (11 residues) and M. brevicollis (17 residues), but shorter in [...] S. rosetta (2 residues)."
That seems quite a lot of variation with only M. brevicollis having a string of pure histidines. In contrast the sequences are around the metal binding segment are quite similar.