The “a-operon” of E.coli is a unit of regulation comprising the following known genes, mostly encoding ribosomal proteins (In order of transcription, and with their products named in brackets): rpsm (S13), rpsK (S11), rpsD (S4), rpoA (a-subunit of RNA polymerase), rplQ (L17). There is evidence that S4 tightly regulates all of these genes, except rpoA, by repressing translation of the polycistronic mRNA. Binding of S4 to the S13 start-site is thought to regulate the first three genes. We have extended the‘rpsD–rpoA’ sequences previously determined by others, to include allrpoA and rplQ. The rpoA–rplQ intercistronic region shows strong primary, and potential secondary structural homologies with the S4-binding sites on 16S rRNA and S13 mRNA. We suggest that S4 represses L17 translation directly.