Involved in the de novo purine biosynthesis. Catalyzes the transfer of formate to 5-phospho-ribosyl-glycinamide (GAR), producing 5-phospho-ribosyl-N-formylglycinamide (FGAR). Formate is provided by PurU via hydrolysis of 10-formyl-tetrahydrofolate. ATP + formate + N(1)-(5-phospho-beta-D-ribosyl)glycinamide = ADP + H(+) + N(2)-formyl-N(1)-(5-phospho-beta-D-ribosyl)glycinamide + phosphate Purine metabolism; IMP biosynthesis via de novo pathway; N(2)-formyl-N(1)-(5-phospho-D-ribosyl)glycinamide from N(1)-(5-phospho-D-ribosyl)glycinamide (formate route): step 1/1. Homodimer. Belongs to the PurK/PurT family.