Met-His-Hygromycin

[Tadahide Izumi]

We are designing a promoterless gene targeting vector by
introducing promoter(-) hygromycinB gene at the translation 
initiation site of the gene of interest.
Initially, we thought of introduction of an NdeI site (catATG), but
this will change the 3 bp sequence just upstream to the initiation codon.

Instead of it, ...atgCATATG... can be introduced with putting two extra amino
acid residues, Met-His, at the N-terminus of the hygromycinB.
Would these 2 residues affect the activity of the enzyme?
Personally I don't think it's likely to have a huge effect by this, but is
there any references that I can make sure of it?
Thanks.