Involved in monoterpene (C10) biosynthesis. The major product is myrcene followed by minor amounts (1.2%) of the cyclic monoterpene limonene. (2E)-geranyl diphosphate = beta-myrcene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Optimum temperature is 40 degrees Celsius. Active up to 55 degrees Celsius. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsb subfamily.