Mitotic instability of integrated plasmids in Penicillium chrysogenum transformants

Transformation vectors based on the Streptoalloteichus hindustanus phleomycin-resistance gene placed under the control of either the Penicillium chrysogenum trpC or pcbC promoters were constructed (plasmids pGS1 and pGS7 respectively). Up to 100 transformants per microgram of DNA were obtained with pGS7 in P. chrysogenum strain P2. In order to follow the expression of additional penicillin biosynthetic genes introduced by transformation, a pcbC::lacZ gene fusion was introduced into pGS1, generating pGS6. Southern analysis of three pGS6 transformants indicated that the plasmid was integrated in tandem arrays. Revertants which had lost the exogenous beta-galactosidase activity, were detected for each transformant after several cycles of subculture on non-selective medium. Southern analysis indicated that the different phenotypes obtained resulted from the loss of part or all of the integrated plasmid copies.