Independent Grid projects have developed their own solutions to Information Services. These solutions are not interoperable with each other, target vastly different systems and address diverse sets of requirements. To address these challenges, we designed a novel architecture for a Grid Information Service that provides unification, federation and interoperability of major grid information services. The proposed approach forms an add-on information system that interacts with the local information services and assembles their metadata instances. We introduce a prototype implementation and present its evaluation. The results indicate that the proposed approach achieves unification and federation of custom implementations of grid information services with negligible processing overheads.