TRANSLATIONAL COUPLING AND LIMITED DEGRADATION OF A POLYCISTRONIC MESSENGER MODULATE DIFFERENTIAL GENE-EXPRESSION IN THE PARD STABILITY SYSTEM OF PLASMID R1

The parD stability system of plasmid R1 is an auto-regulated operon co ntaining two genes, kis and kid, that code, respectively, for a killer protein (Kid) and for an antagonist of Kid action (Kis protein). A po lycistronic transcript and a shorter mRNA, coding only for Kis and end ing in a stem-loop sequence, have been identified as the main parD tra nscripts in cells carrying a derepressed parD operon. In this communic ation 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 ext racts of Escherichia coli, we demonstrate that the short kis mRNA orig inates from limited degradation of the bicistronic parD transcript and that the stem-loop structure within the 5' end of the kid gene is spe cifically required for the formation of this short transcript. In vivo experiments show that synthesis of Kis is required for efficient synt hesis of Kid. These data indicate that RNA processing and translationa l coupling are important mechanisms that modulate the differential exp ression of the two genes, kis and kid, in the bicistronic parD operon.