A light microscope study of linker histone distribution in rat metaphase chromosomes and interphase nuclei

Several subtypes of the linker histone H1 are present in normal rat kidney epithelial cells (NRK-52E). Although H1 is essential in nucleosome and chromatin packaging or condensation, the unique functions of these very basic proteins are largely unknown. There has been much speculation on the role of each H1 variant on developmentally regulated or tissue specific gene expression. We have examined the global distribution of several H1 subtypes on metaphase chromosomes in an attempt to uncover large-scale differences in chromatin condensation. Polyclonal antibodies raised against HPLC-purified rat H1 subtypes revealed a pattern much like G or Q bands for all H1 variants tested on chromosomes harvested with either aqueous or organic spreading methods. H1^o, a less abundant form of H1, may be associated with terminally differentiated or senescent cells. In cultures treated to induce higher levels of H1^o there were no visible differences at the light microscope level in the antibody banding pattern between induced and noninduced cells. The distributions of H1 subtypes on chromosomes may be visible in different tissues when viewed at higher magnifications. While chromosome patterns were consistent with the antibodies tested, the interphase nuclei displayed clear differences. An epitope specific for anti-H1A antibody is present in the nuclear envelope and is possibly used for chromosomal location or anchorage. Anti-H1B antibody did not specifically label the nuclear envelope, nor did anti-H1^o antibody. Highly concentrated regions of H1^o surround the nucleoli, possibly indicating a cluster of genes that are poised for transcription.