The sentence is correct. The 'constructions' is the object and 'it' is the subject of that clause. Subject and verb must agree, not necessarily the object. You might be confusing it with: Here are things... There are things... In these cases, the subject comes after the main verb and here/there are auxiliary/helping words (sorry, I can't remember the technical name).

I think it should read "It is the construction which contributes to the major part of the meaning."