This study focused on the characterization of the gene rsmS in Erwinia carotovora subsp. atroseptica (Eca). Eca is a phytopathogenic Gram-negative bacteria, producing virulence factors, such as plant cell wall degrading enzymes {PCWDEs), which enable the initiation and maintenance of host plant invasion. rsmS was shown to encode a short polypeptide, and it has orthologues in many Gramnegative agriculturally and medically relevant bacteria. Plate and spectrophotometry-based assays and microarray studies showed that RsmS suppressed the transcription and activity of several PCWDEs, the transcription of type one, two and six secretion systems and of some of their substrates. RsmS also suppressed general virulence exhibited by Eca in potato tubers, and on the quorum sensing signaling molecule N-{3-oxohexanoylh-homoserine lactone. In many ways, RsmS acted in a similar fashion to RsmA, r a well characterized suppressor of virulence determinants (such as PCWDEs) and quorum sensing found in a wide variety of Gram-negative bacteria. RsmA activated rsmS transcription, and the transcription of both rsmS and rsmA was shown to be upregulated with increasing temperature. Thus, it is possible that RsmA exerts some of its inhibitory effects on virulence determinants through the positive regulation of RsmS. rsmS was shown to be transcribed from a promoter shared with its upstream neighbour, priC. It may also be transcribed from a promoter located within the priC open reading frame, but this result remains to be confirmed with future experiments. Although the genomic arrangement of priC and rsmS is conserved in almost all bacteria with an rsmS orthologue, no further regulatory connection was shown between their protein products. Interestingly, when rsmS was overexpressed without a 225 bp region located within priC, PCWDE activity was increased, in all mutant strains of Eca tested. The reason behind this dominant negative phenotype remains to be fully elucidated by future experiments. Both microarray and plasposon mutagenesis studies identified several genes coding for factors that are regulated by, or regulate, RsmS. It will require further analysis and experiments to unveil which one of these actually interacts with RsmS (directly or indirectly). Such genes include CRISPR-like genes, members of nitrogen metabolism pathways and membrane transport. 4