Friday, July 25, 2008

The Role of Plasmid-encoded H-NS-like Protein

"An H-NS-like stealth protein aids horizontal DNA transmission in bacteria" Doyle M et al., Science 315: 251-252 (2007)

H-NS is one of the abundant DNA-binding proteins that are found in gram-negative bacterial cells. H-NS is known to bind to A+T-rich sequences and regulate expression of a large number of chromosomal genes (Fang FC and Rimsky S, 2008) . Recently, it has become clear that some narrow-host-range plasmids derived from gram-negative bacteria encode H-NS-like proteins. Yet, their roles were still obscure.

So far, it has been shown that Sfh, an H-NS paralog encoded by IncHI1 group plasmid pSf-R27, interacts with host H-NS, and both proteins are functionally exchangeable (Deighan P et al., 2003; Beloin C et al., 2003). People thus might think that plasmid-encoded H-NS-like proteins influence global gene expression of host bacteria. Interestingly, the results shown in this article look to be in the contrary: the absence of Shf disturbed global gene expression of transconjugants. In this article, authors proposed that plasmid-encoded H-NS is a stealth protein that allows host bacteria to carry A+T-rich plasmids with minimal effect on global gene expression and "fitness", by preventing plasmids from titrating cellular pool of H-NS.

Authors introduced pSf-R27, with or without the sfh gene, from original host Shigella flexeneri into Salmonella Typhimurium, and analyzed the transcriptome as well as several phenotypes of transconjugants. Interestingly, the transfer of wild-type pSf-R27 resulted in a few change in the recipient, but transfer of pSf-27Δsfh resulted in the drastic changes in expression of a wide range of genes. Noteworthy phenotypes of the recipient carrying sfh mutant were increased resistance to UV, increased virulence (persistence in macrophage) and reduced motility. These phenotypes are reminiscent of the chromosomal hns mutant (Navarre WW et al., 2006). Authors then showed that the sfh mutation significantly reduced the fitness of recipient (this phenotype was completely complemented by supplying Sfh in trans from another plasmid). To figure out if the reduction of fitness resulted from the titration of "host" H-NS by A+T-rich sequence on the plasmid, authors constructed a pUC18 derivative that carried chromosome-derived A+T-rich DNA fragment and introduced it into the recipient cells, instead of pSf-R27Δsfh. The recipient that carries the pUC18 derivative caused reduction in the fitness, and this reduction was complemented in the presence of Sfh, as in the case of pSf-R27.

Based on these observations authors proposed that sfh is a "stealth" gene that allows the A+T-rich pSf-R27 to invade a new bacterial host with a minimal impact on global gene expression patterns and fitness. They added that the positive effects of sfh on the fitness can be applied to biotechnology to construct more stable cloning vectors.

Doyle M, Fookes M, Ivens A, Mangan MW, Wain J, Dorman CJ.
An H-NS-like stealth protein aids horizontal DNA transmission in bacteria.
Science 2007, 315:251-2.

Fang FC, Rimsky S.
New insights into transcriptional regulation by H-NS.
Curr. Opin. Microbiol. 2008, 11:113-20.

Deighan P, Beloin C, Dorman CJ.
Three-way interactions among the Sfh, StpA and H-NS nucleoid-structuring proteins of Shigella flexneri 2a strain 2457T.
Mol. Microbiol. 2003, 48:1401-16.

Beloin C, Deighan P, Doyle M, Dorman CJ.
Shigella flexneri 2a strain 2457T expresses three members of the H-NS-like protein family: characterization of the Sfh protein.
Mol. Genet. Genomics 2003, 270:66-77.

Navarre WW, Porwollik S, Wang Y, McClelland M, Rosen H, Libby SJ, Fang FC. Selective silencing of foreign DNA with low GC content by the H-NS protein in Salmonella.
Science. 2006 Jul 14;313(5784):236-8.

Hirokazu Yano (Ph. D.)
University of Idaho

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