Catalyzes the sequential condensation of isopentenyl diphosphate (IPP) in the cis configuration with (2Z,6E)-farnesyl diphosphate (Z-FPP or EZ-FPP) generating the 50 carbon product trans,polycis-decaprenyl diphosphate. When (2E,6E)-farnesyl diphosphate (E-FPP or EE-FPP) is used in vitro, both primary products decaprenyl diphosphate and heptaprenyl diphosphate are synthesized. It is probably due to the fact that M.smegmatis synthesizes both (2E,6E,10E)-geranylgeranyl diphosphate (EEE-GGPP) and (2E,6E,10Z)-geranylgeranyl diphosphate (EEZ-GGPP). Can also accept many different allylic substrates, including E-geranyl diphosphate (E-GPP), neryl diphosphate (NPP), and all-trans-geranyl-geranyl diphosphate. (2Z,6E)-farnesyl diphosphate + 7 isopentenyl diphosphate = (2Z,6Z,10Z,14Z,18Z,22Z,26Z,30Z,34E)-decaprenyl diphosphate + 7 diphosphate (2E,6E)-farnesyl diphosphate + n isopentenyl diphosphate = di-trans,poly-cis-polyprenyl diphosphate + n diphosphate Binds 2 magnesium ions per subunit. Homodimer. Belongs to the UPP synthase family.