Catalysis of the reaction: dimethylallyl diphosphate + isopentenyl diphosphate = diphosphate + geranyl diphosphate.
Source:EC:2.5.1.1
Comment
Note that this is the first step in the formation of farnesyl diphosphate. The second step is 'geranyltranstransferase activity ; GO:0004337'. Consider also annotating to the biological process term 'farnesyl diphosphate biosynthetic process ; GO:0045337'.