Catalyzes the transfer of the phosphoribosyl group of 5-phosphorylribose-1-pyrophosphate (PRPP) to anthranilate to yield N-(5'-phosphoribosyl)-anthranilate (PRA). diphosphate + N-(5-phospho-beta-D-ribosyl)anthranilate = 5-phospho-alpha-D-ribose 1-diphosphate + anthranilate Binds 2 magnesium ions per monomer. Amino-acid biosynthesis; L-tryptophan biosynthesis; L-tryptophan from chorismate: step 2/5. Homodimer. Belongs to the anthranilate phosphoribosyltransferase family. Extended N-terminus.