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From 2006.igem.org
Fis (factor for inversion stimulation). Fis is a small basic DNA-binding protein that was identified in E. coli as a factor involved in site-specific DNA recombination. Several lines of evidence indicate that Fis also participates in other processes of DNA functions such as transcription of the growth-related genes and DNA replication. The intracellular level of Fis protein in exponential growth phase cells of E. coli W3110 was found to be about 60,000 molecules per cell at maximum, but thereafter it decreased to undetectable levels in the stationary phase. Fis is the most abundant nucleoid-associated protein in growing E. coli cells. The growth-dependent change in the Fis level is in good agreement with the observations that Fis is needed for transcription of growth-related genes, such as those for rRNA and tRNA, and for DNA replication.
Measurement of the intracellular Fis level agrees well with the observations that the Fis level in E. coli MC1000 growing under various conditions fluctuates within a range of less than 100 in the stationary phase to over 50,000 molecules per cell in the early-exponential growth phase. The regulation of Fis is essentially the same in Salmonella typhimurium, even though the autoregulation is less efficient. The timing of the Fis peak occurs prior to the first cell division of cell growth after recovery from the stationary phase. If the DNA-bound Fis remains a homodimer, Fis may bind every 200 to 300 bp of DNA, on average, along the genome DNA in growing cells of E. coli (but the Fis sites are not regularly distributed along the genome DNA). Highly expressed Fis represses its own synthesis by binding to the fis promoter region. Upon entry into the stationary phase, Fis synthesis is switched off, resulting in a decrease in intracellular level by 500- to 1,000 fold.
The crystal structure of Fis adopts a helical structure with four helices in each subunit. The two monomers are interlocked with extensive interactions between them. The helix-turn-helix (HTH) DNA binding motif constitutes the main component of the DNA binding surface. Fis induces large bends in DNA and that the overall bending angle is dependent on the sequences flanking both sides of a 15-base pair core binding site. Based on a computer model of the Fis/DNA complex, Cu(+2)-mediated site-specific cleavage experiments identified potential protein-DNA contacts that are responsible for the observed DNA bending. It is apparent that an induced fit mechanism is involved in the Fis/DNA recognition process.
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