How do I retrieve electronic annotation from Uniprot and GO records that relates to molecular function?
The endonuclease is controlled vocabulary. In this case, it looks like GO actually cites UniProt as the source for the endonuclease keyword.

With the Uniprot url, whenever you want to see where the information comes from for the annotation in Uniprot, type /publications after the ID. For example: http://www.uniprot.org/uniprot/Q13283/publications