The Escherichia coli CsrB and CsrC small RNAs are strongly induced during growth in nutrient-poor medium.

The carbon storage regulatory (Csr) system is a complex network controlling various phenotypes in many eubacteria. So far, the external conditions by which the system is regulated are poorly understood. Here we show that the expression of the two noncoding small RNAs CsrB and CsrC in Escherichia coli is strongly increased in cultures grown in minimal medium. Addition of tryptone, casamino acids or a mixture of amino acids to a culture grown in minimal medium led to a rapid reduction in the levels of CsrB. Based on this we propose that the expression of the Csr sRNAs is controlled by the amino acid availability in the growth medium.