We have investigated the nuclear transport of the replacement histone H1(0) and have searched for its nuclear localization sequence (NLS). The lysine-rich H1(0) histone differs from the other H1 histones with respect to its mode of expression and to the processing of the respective mRNA. Using the digitonin-permeabilized cell import assay we demonstrate that H1(0) is transported into the nucleus in an energy- and temperature-dependent manner. In competition experiments we show that the transport of H1(0) from the cytoplasm into the nucleus is competed by the SV40 T-antigen-NLS-peptide coupled to HSA, an established substrate of the importin pathway. In transfection studies we have expressed in HeLa cells a series of plasmid constructs containing different fragments of the coding region of the H1(0) histone gene that were fused to the beta-galactosidase gene, and we have determined the subcellular localization of each fusion protein. The results show that H1(0) contains multiple transport-competent sequence elements that can function as NLS and that H1(0) meets the requirements for a transport into the nucleus by an importin-dependent pathway.
Copyright 1998 Academic Press.