This fragment is prepared from the pBR322-SV40
hybrid plasmid, in which the tetracycline-resistance gene between the HindIII and PvuII sites in the pBR322 is replaced with the SV40 PvuII-HindIII (map unit, 0.71-0.86) fragment that contains a single KpnI and HpaI site, the former of which provides the ends for poly(dT) tailing with TdT and the latter provides the digestion site for the removal of one poly(dT) tail from the plasmid DNA.