Figure 7.

Model for self-limited regulation by a small pairing RNA. Normal growth: Only small amounts of the small RNA (sRNA) may be expressed; these will be used quickly, clearing them from the cell. Hfq may bind to some portion of the target mRNAs. Step 1: In response to a stress or other regulatory signal (Fe limitation for RyhB), synthesis of the sRNA rapidly increases and is bound by Hfq (yellow ring). Step 2: The small RNA pairs with its target. Pairing is frequently but not always near the start of translation; in some cases, two or more short (7-9 nucleotides) regions of complementarity are used (for review, see Gottesman 2002). Both small RNA and target mRNA are degraded rapidly in an RNase E-dependent process (scissors). As long as the small RNA continues to be synthesized at a high rate, it will accumulate, and target messages will be destroyed. Step 3: After the stress signal is lost, synthesis of the small RNA will drop to basal levels. The sRNA that has accumulated will be degraded as it is used, allowing reaccumulation of the target mRNAs (Normal growth).