ISWI chromatin remodeling: one primary actor or a coordinated effort?

The ISWI family of ATP-dependent chromatin remodelers regulates transcription of coding and noncoding RNA by mobilizing nucleosomes and controlling the length of linker DNA separating nucleosomes (spacing). Nucleosome movement is tightly coupled to the DNA translocation activity of the helicase domain in the catalytic subunit. There may be other domains besides the helicase domain needed to move DNA in and out of nucleosomes. The C terminus of the ISWI catalytic subunit with the conserved HAND, SANT, and SLIDE domains may be involved in nucleosome spacing. There are several models of how the C terminus may facilitate in ISWI remodeling such as regulating the activity of the helicase domain and causing the helicase domain to translocate more efficiently on DNA or to enhance its selectivity for nucleosomes. Another possibility is that domains like SLIDE promote linker DNA entering into nucleosomes in a coordinated manner with the helicase domain.