An original model for control of mini-F replication is proposed. In this model a 29,000 dalton protein (29 kd) is predicted to work both as an autorepressor and after modification, perhaps by forming oligomers, as an initiator. Control by the autorepression system is augmented by a second regulatory system to further reduce the frequency of replication. This system appears to be a pseudoorigin that competes with the true origin for the binding of the 29 kd protein derivative postulated to act as an initiator. Both the pseudo and true origin contain identical 19 base pair direct repeats. These repeats were identified earlier as incompatibility determinants. Together these components form the essential genes of mini-F replication. The model serves as a basis for specific predictions about the binding sites of 29 kd protein (repressor) and 29 kd derivatives (initiator). These predictions will be examined after purifying the 29 kd protein using a variety of biochemical and immunological assays. The model also allows specific predictions to be made about mini-F replication and incompatibility as a function of 29 kd cellular concentration. Biochemical-genetic complementation tests are proposed to examine these predictions. A second origin, unrelated to the one discussed above by virtue of totally different DNA sequence, exists in mini-F. This second origin is dispensible; however, when present it can be dominantly used in preference to the first origin located in the essential genes. Moreover, the second origin requires the presence of DNA region harboring the essential genes in order to function. A series of tests are proposed that will allow identification of known genetic components in the essential genes that are required for usage of the second origin of replication. The long-term goals of these efforts are to understand the molecular basis for the biochemical-genetic processes of plasmid maintenance. The mini-F plasmid system is related by incompatibility to many plasmids that determine antibiotic resistance and microbial virulence. Therefore, fundemental knowledge about mini-F maintenance genes is directly applicable to medically important plasmids.
