Each Caulobacter cell division yields daughter cells that differ from one another both structurally and functionally. By focusing on the biogenesis of the polar flagellum and the proteins of the chemosensory system, several laboratories have now defined an extensive network of genes whose temporal expression is controlled in the predivisional cell. The differential turn-on of these genes contributes to the generation of asymmetry in the predivisional cell in that the products of these genes are targeted to specific cellular locations. To define the mechanisms that mediate this temporal and spatial control, fla genes whose products are not known were accessed by the insertion of transposon-carried drug resistance markers. The transposons were altered so that upon insertion into the chromosome, transcription fusions are formed in which the promoter regions of fla genes drive the expression of the downstream promoter-less drug resistance genes. Assays of the differential placement of the promoter-less drug resistance proteins (encoded within the interrupted fla genes) allow us to determine whether the positioning of the fla gene products is controlled by signal sequences in their proteins, by specific mRNA-targeting sequences in the 5'-regulatory regions of these genes, or by specific transcription from only one of the two newly replicated chromosomes in the predivisional cell.