Three-way interactions among the Sfh, StpA and H-NS nucleoid-structuring proteins of Shigella flexneri 2a strain 2457T.

Shigella flexneri 2a strain 2457T has been found to express Sfh, a new member of the H-NS-like family of nucleoid-structuring proteins. With H-NS and its paralogue, StpA, this brings to three the number of these proteins expressed in this bacterium. This raises the possibility that three-way interactions may occur in S. flexneri among these proteins and between the proteins and each other's genes. Such three-way interactions among H-NS-like proteins have not been described previously. The expression of the sfh, stpA and hns genes was studied at the transcriptional and post-transcriptional levels. The Sfh protein displays growth phase-dependent regulation that distinguishes it from both H-NS and StpA. Like H-NS and StpA, Sfh can bind to its own promoter region, it negatively autoregulates transcription of its own gene, and when overexpressed all three proteins cross-repress transcription of each other's genes. The presence of highly conserved oligomerization domains within these molecules suggested the possibility of protein-protein interactions. Like H-NS and StpA, the purified Sfh protein forms homodimers in solution. Using the yeast two-hybrid assay we show that each of the three proteins also forms homodimers in vivo and, additionally, each protein can form heterodimers with either of its homologues. This raises the possibility that Sfh may modulate the activities of H-NS and StpA, and vice versa.