Linker Histone Subtypes Differ in Their Effect on Nucleosomal Spacing In Vivo

Abstract

Linker histone H1 is located on the surface of the nucleosome where it interacts with the linker DNA region and stabilizes the 30-nm chromatin fiber. Vertebrates have several different, relatively conserved subtypes of H1; however, the functional reason for this is unclear. We have previously shown that H1 can be reconstituted in Xenopus oocytes, cells that lack somatic H1, by cytosolic mRNA injection and incorporated into in vivo assembled chromatin. Using this assay, we have expressed individual H1 subtypes in the oocytes to study their effect on chromatin structure using nucleosomal repeat length (NRL) as readout. We have compared chicken differentiation-specific histone H5, Xenopus differentiation-specific xH10 and the somatic variant xH1A as well as the ubiquitously expressed human somatic subtypes hH1.2, hH1.3, hH1.4 and hH1.5. This shows that all subtypes, except for human H1.5, result in a saturable increase in NRL. hH1.4 results in an increase of approximately 13–20 bp as does xH10 and xH1A. chH5 gives rise to the same or slightly longer increase compared to hH1.4. Interestingly, both hH1.2 and hH1.3 show a less extensive increase of only 4.5–7 bp in the NRL, thus yielding the shortest increase of the studied subtypes. We show for the first time in an in vivo system lacking H1 background that ubiquitously expressed and redundant H1 subtypes that coexist in most types of cells of higher eukaryotes differ in their effects on the nucleosomal spacing in vivo. This suggests that H1 subtypes have different roles in the organization and functioning of the chromatin fiber.