Reciprocal transcriptional and posttranscriptional growth-phase-dependent expression of sfh, a gene that encodes a paralogue of the nucleoid-associated protein H-NS.

The IncHI1 self-transmissible plasmid pSf-R27 from Shigella flexneri 2a strain 2457T harbors sfh, a gene that codes for a protein with strong amino acid sequence homology to the global transcription regulator and nucleoid-associated protein H-NS and to its paralogue, StpA. Previously, we discovered that the expression of sfh mRNA is growth phase dependent such that in cultures growing in Lennox broth at 37 degrees C, the transcript is readily detectable in the early stages of exponential growth but is not detectable at the onset of stationary phase. In contrast, the Sfh protein is poorly expressed in early-exponential growth when sfh mRNA is abundant whereas it is expressed to a high level in early stationary phase, when sfh transcript expression is low (P. Deighan, C. Beloin, and C. J. Dorman, Mol. Microbiol. 48:1401-1416, 2003). This unusual pattern of reciprocal mRNA and protein expression is not due to growth phase-dependent effects on either mRNA or protein stability, nor is it due to the known abilities of the Sfh, StpA, and H-NS proteins to influence sfh gene expression. Instead, our data point to a blockade of sfh mRNA translation in early-exponential growth that is relieved as the culture enters the stationary phase of growth. Replacing the 5' end and translation initiation signals of the sfh mRNA with heterologous sequences did not alter the growth phase-dependent expression of the Sfh protein, suggesting that growth phase control of translation is intrinsic to another component of the message.