MspA is the major porin of Mycobacterium smegmatis and is important for diffusion of small and hydrophilic solutes across its unique outer membrane. The start point of transcription of the mspA gene was mapped by primer extension and S1 nuclease experiments. The main promoter driving transcription of mspA was identified by single point mutations in lacZ fusions and resembled σA promoters of M. smegmatis. However, a 500-bp upstream fragment including PmspA in a transcriptional fusion with lacZ yielded only low β-galactosidase activity, whereas activity increased 12-fold with a 700-bp fragment. Activation of PmspA by the 200-bp element was almost eliminated by increasing the distance by 14 bp, indicating binding of an activator protein. The chromosomal mspA transcript had a size of 900 bases and was very stable with a half-life of 6 minutes, whereas the stabilities of episomal mspA transcripts with three other 5′ untranslated region (UTRs) were three- to sixfold reduced, indicating a stabilizing role of the native 5′ UTR of mspA. Northern blot experiments revealed that the amount of mspA mRNA was increased under nitrogen limitation but reduced under carbon and phosphate limitation at 42°C in stationary phase in the presence of 0.5 M sodium chloride, 18 mM hydrogen peroxide, and 10% ethanol and at acidic pH. These results show for the first time that M. smegmatis regulates porin gene expression to optimize uptake of certain nutrients and to protect itself from toxic solutes. Copyright © 2007, American Society for Microbiology. All Rights Reserved.