Expression of hns and stpA in Salmonella enterica servoar Typhimurium

Abstract

H-NS-like proteins are widespread amongst Enterobacteriaceae and other Gramnegative bacteria. They are small (15 kDa) nucleoid associated proteins involved in many cellular processes such as replication, transposition, chromosome compaction and global gene regulation. Salmonella enterica serovar Typhimurium possesses two such proteins, H-NS and StpA, encoded chromosomally by the hns and stpA genes, respectively. Experimental evidence from both S. Typhimurium and E. coll has shown H-NS and StpA to be involved in many of the same regulatory interactions and indeed to bind at many of the same loci across the genome. These functional similarities probably reflect their structural similarity, with each having a C-terminal nucleic acid binding domain, a flexible linker region at mid-protein and an N-terminal oligomerization domain which is used for both self- and hetero-associations. Despite such similarities in terms of structure and function, each protein exhibits a highly individual pattern of expression. H-NS protein levels are constant throughout growth while transcription of hns mRNA is coupled with chromosome replication and is maximal throughout exponential phase. StpA protein and stpA mRNA on the other hand peak in early exponential phase before declining to a steady state level thereafter. By interchanging the open reading frames (ORFs) of hns and stpA we aimed to assess the contributions made by the regulatory region, ORF and chromosomal location of each gene to these individual expression patterns. In making these exchanges an hns-like pattern of expression was conferred on stpA and vice versa. Subtle alterations in hns and StpA expression, however, were observed at both the mRNA and protein levels compared to wild-type cells. Interestingly, irrespective of which ORF was under its control, transcriptional activity via the stpA promoter was significantly greater than that observed for the hns promoter. This, however, is not reflected at the protein level and may reflect differences in mRNA stability. Finally, in making these regulatory changes, although subtle, the observed variations in hns and stpA expression at the mRNA and protein levels places the bacterium at a competitive advantage relative to wild-type at 37°C and 42°C. The mutants with exchanged stpA and hns ORFs also exhibit alterations in gene expression throughout their respective regulons that are accompanied by a number of new phenotypes.