Strong mimicry of an rRNA binding site for two proteins by the mRNA encoding both proteins

There are numerous cases where a ribosomal protein autoregulates by binding to its own mRNA, usually explained as the mRNA site mimicking the rRNA site for the protein, although the resemblance is usually quite limited. Here an mRNA segment is described that very strongly resembles a segment of 23S rRNA at the primary sequence and secondary structure levels, in a gammaproteobacterial clade comprising Pseudomonadaceae and certain marine bacteria. Moreover this segment from rRNA is known to bind either or both of two proteins, L4 and L24, that are encoded downstream of the mRNA segment. Analysis of Pseudomonas aeruginosa gene regulatory signals in the region suggests that the L4-encoding operon extends to include the L24 gene. Thus two proteins may co-autoregulate their shared operon, with interesting combinatorial possibilities. This implies that two independent autoregulatory mechanisms have arisen for this operon in the Gammaproteobacteria, since the L4-encoding operon of Escherichia coli does not include the L24 gene and is autoregulated by L4 protein alone at a different site that bears weak resemblance to its binding site in 23S rRNA.