A set of artificial circular plasmids, named plasmoids, was constructed. They are about 1 kb in size and consist of a 178 bp λdv minimal DNA replication origin (ori) which has four direct repeats and the A+T-rich region conferring polarity to the ori fragment, a 775 bp DNA segment that codes for the CAT amino acid sequence and the 99 bp lac promoter (plac). They carry no other functional genes or genetic sites. The constructions involved various combinations of relative orientations of these components. These molecules do not replicate in vivo because they lack genes coding for initiation proteins, but they do replicate in an in vitro system (Fuller et al. 1981), and can be used for studies of interactions between transcription and replication. In these plasmoids, major transcription starts from the strong plac, and some weak unscheduled transcription starts from several other initiation sites. The major RNA synthesis was found to interfere with the unscheduled RNA synthesis, which was occurring on the opposite strand. The most active replication took place when the major RNA synthesis went through the λ origin region in the direction which occurs naturally in the λ genome. Under these conditions, DNA synthesis going against such transcription was less than that going along with the major transcription. When RNA synthesis through the λ origin region was in the opposite direction, DNA synthesis in the same direction was about half of that observed in the above case, whereas that going against transcription was very weak. Based on these observations, this paper discussed the interactions between the two transcription systems and between transcription and DNA synthesis.
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