Influence of fd gene 2-protein and the viral replication origin on the compatibility of pfd-plasmids

Plasmids with the replication origin of bacteriophage fd, the pfd-plasmids, were investigated for compatibility in E. coli cells expressing fd gene 2-protein. This was measured by transformation of Ca-treated cells with and without a residing pfd-plasmid. When the two plasmids contained the complete intergenic region of bacteriophage fd, they were fully compatible in contrast to the situation in which at least one plasmid had a shortened origin for viral strand replication. This incompatibility effect was partially compensated for by a pfd-plasmid with a short origin and with the fd gene 2. The fd replication origin on a colEl plasmid did not affect compatibility in polA+ cells indicating its idling in the presence of the colEl origin. It can be concluded that a short replication origin requires high amounts of gene 2-protein in contrast to the long origin. Accumulation of replication intermediates severely interferes with host cell metabolism.