A survey of H1o-and H5-like protein structure and distribution in higher and lower eukaryotes

A survey of H1o and H5-like proteins has been conducted through a range of higher and lower eukaryotic species. All mammals examined possessed H1o proteins, although in variable amounts, and the protein's structure was well conserved, though not invariant. The testis-specific histone Hlt (from rat) did not have an H1o-like structure and it appears that H1o does not occur in spermatocytes in any form. The results also show that Xenopus laevis contains H1o-like proteins, but lower, non-vertebrate eukaryotes (a crustacean, two fungi and a plant tissue) do not possess H1o or H5 proteins. The evidence suggests that H1o and H5 proteins may be considered as belonging to one family, distinct from H1 types. This H1o/H5 family may well be 'replacement histone' variants of H1. The results do not support suggestions of roles such as repression of DNA synthesis or of transcription for H1o/H5 proteins.