Ribosomal protein S1 is shown to interact with the non-coding RNA DsrA and with rpoS mRNA. DsrA is a non-coding RNA that is important in controlling expression of the rpoS gene product in Escherichia coli. Photochemical crosslinking, quadrupole-time of flight tandem mass spectrometry, and peptide sequencing have identified an interaction between DsrA and S1 in the 30S ribosomal subunit. Purified S1 binds both DsrA (K-obs similar to 6 x 10(6) M-1) and rpoS mRNA (K-obs similar to 3 x 10(7) M-1). Ribonuclease probing experiments indicate that S1 binding has a weak but detectable effect on the secondary structure of DsrA or rpoS mRNA. (c) 2006 Elsevier Inc. All rights reserved.