The high molecular weight chromatin proteins of winter flounder sperm are related to an extreme histone H1 variant

Abstract

Unlike mammals, birds, and most other fishes, winter flounder completes spermatogenesis without replacing its germ cell histones with protamines. Instead, during spermiogenesis, these fish produce a family of high molecular weight (80,000-200,000) basic nuclear proteins (HM(r)BNPs) that bind to sperm chromatin containing the normal complement of histones. These large, basic proteins are built up of tandem iterations of oligopeptide repeats that contain phosphorylatable DNA-binding motifs. Although the HM(r)BNPs have no obvious homology to histones, protamines, or other sperm-]specific chromatin proteins, we report here the isolation of a clone (2B) from a winter flounder genomic DNA library that establishes a link between the HM(r)BNPs and histone H1. The 2B sequence contains an open reading frame, which, when conceptually translated, encodes a 265-residue protein. At its N terminus the translation product contains numerous simple repeats that match the oligopeptides contained within the HM(r)BNPs. Unexpectedly, the C terminus of the putative protein shows 66% identity and 76% conservation to the histone H1 globular domain. This connection suggests that the HM(r)BNPs may have originated from the extended N- terminal tail region of a testis-specific, H1-like linker histone.