[The big world of the small RNAs].

In the last ten years a great number of RNAs have been discovered that function by regulating gene expression and do not code for a protein. This RNAs are named ncRNAs or sRNAs (non coding RNAs or small RNAs). More than 70 sRNAs have been documented in Escherichia coli and function by inhibiting/ promoting translation or degradation, some have intrinsic catalytic properties and others modify the activity of a protein. The mechanisms of action of some of them will be reviewed as well as the efforts to find new sRNAs by systematic oriented searches.