Histone-like nucleoid-structuring protein

The nucleoid -associated protein H -NS (English histone -like nucleoid structuring protein ) is an important Genregulatorprotein in enterobacteria, with mostly repressing effect. It plays a key role in adapting to changing environmental conditions and during stress. In addition, H -NS protects the bacterium from foreign, potentially harmful DNA. In E. coli 200-300 genes are under his control and it has about 20,000 molecules per cell, one of the most common DNA-binding proteins.

Repressing effect

Although H -NS binds to a DNA-binding protein not specific to DNA, however, can be very specific expression of certain genes suppress ( silencing ). There oligomerized and preferentially binds only weakly binds specifically to AT-rich DNA sequences. If these AT-rich sequences in the vicinity of a promoter, the RNA polymerase can not start the transcription and the gene is off. Frequently bind H -NS molecules both before ( upstream) and after ( downstream) the promoter, so that a DNA -H -NS - DNA bridge (see chart) is "trapped" in the loop, the RNA polymerase. Is this case, the gene is also repressed.

Although H -NS binds very specific to AT-rich sequences, it is still to control specifically able genes. This fact is explained by the fact that in the activation of genes that are repressed by H -NS, many different, highly specific proteins play a role that can displace H -NS by different mechanisms. Therefore, the activation of the H -NS repressed gene by a de - repression or anti- silencing is called.

Protective function

Another key function of H - NS is the repression of foreign, recorded by horizontal gene transfer DNA ( xenogeneic silencing ). This feature protects the bacterium from harmful effects by phages inject their DNA into the bacterium as a host cell, which makes this sooner or later perish. In general, the recognition by the increased AT / GC ratio of the foreign DNA occurs.