When the latter half of the hup gene encoding a histone-like protein HSl of Streptomyces lividans TK24 was replaced by the kanamycin resistance gene, the hup mutant EY1 grew slowly in liquid medium and this delay was overcome by introduction of the complete hup. EY1 sporulated normally on solid medium, with no serious defects as observed in hupAB mutants of Escherichia coli. Therefore, HSl probably has a role in growth in the presence of liquid medium and this organism may possess another histone-like protein with functions overlapping those of HSl. We cloned the hup2 gene encoding another histone-like protein HSl2, which has two motifs of prokaryotic histone-like protein and eukaryotic histone H1. The amount of HSl2 increased in EY1, determined by western blotting analysis using an anti-His-tagged HSl2 polyclonal antibody. We are entertaining the notion that the increased amount of HSl2 partially suppressed the defects caused by the hup mutation.
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