The parD stability system of plasmid R1 is an auto-regulated operon containing two genes, kis and kid, that code, respectively, for a killer protein (Kid) and for an antagonist of Kid action (Kis protein). A polycistronic transcript and a shorter mRNA, coding only for Kis and ending in a stem-loop sequence, have been identified as the main parD transcripts in cells carrying a derepressed parD operon. In this communication we show that both parD mRNAs have a half-life close to 1 min and are present in similar amounts. Using an assay based on cell-free extracts of Escherichia coli, we demonstrate that the short kis mRNA originates from limited degradation of the bicistronic parD transcript and that the stem-loop structure within the 5' end of the kid gene is specifically required for the formation of this short transcript. In vivo experiments show that synthesis of Kis is required for efficient synthesis of Kid. These data indicate that RNA processing and translational coupling are important mechanisms that modulate the differential expression of the two genes, kis and kid, in the bicistronic parD operon.
