MicF

MicF is a small regulatory non-coding RNA found in Escherichia coli and related Enterobacteriaceae. It is about 90 to 100 nucleotides long and functions as a post-transcriptional regulator of outer membrane porin expression, most notably repressing the translation of ompF mRNA and promoting its degradation, thereby reducing OmpF porin levels.

Mechanism: MicF acts by base-pairing with the 5' untranslated region of ompF mRNA around the Shine-Dalgarno sequence, blocking ribosome access. This pairing can also mark ompF mRNA for RNase E–dependent decay. The RNA often requires the Hfq chaperone to facilitate the interaction.

Regulation and physiological role: micF transcription is part of the EnvZ/OmpR two-component regulon. In response to environmental cues such as high osmolarity or envelope stress, micF is upregulated, contributing to decreased OmpF and a remodeled outer membrane. This regulation influences permeability to nutrients and to some antibiotics and toxic compounds.

Conservation and notes: MicF is among the earliest characterized bacterial small RNAs and is conserved among many Enterobacteriaceae; while ompF is its primary validated target, other transcripts have been reported in different strains or genetic backgrounds.