Structure and expression of the highly repetitive histone H1-related spermchromatin proteins from winter flounder

In the late stages of spermatogenesis, winter flounder produce a family of high molecular mass (80-200 kDa) basic nuclear proteins (HM(r)BNPs) that co mbine with the normal complement of histones to produce condensed sperm chr omatin with an increased nucleosomal repeat length. The HM(r)BNPs have a bi ased amino-acid composition in which Arg, Ser, Lys and Pro are abundant bec ause of their presence in many simple peptide repeats. The organization of these repeats was deduced by cDNA cloning. The predominant repeating units are related 26- and 30-amino-acid sequences that in turn are linked by 6-am ino-acid spacers to form 58- and 62-amino-acid repeats. Subsets of these re peats are also present, such as a dispersed 20-amino-acid repeat and a tand em array of nine heptapeptides at the C-terminus. The HM(r)BNPs appear to h ave evolved from an extreme H1 variant that has an N-terminal tail of HMrBN P-like sequence linked to an H1 globular region. Based on sequences of the most abundant HMrBNP cDNAs, and the lack of hybridization between HMrBNP mR NAs and a DNA probe for the H1 globular region, the latter domain appears t o have been lost during expansion and amplification of the HMrBNP-like repe ats. Transcripts of the HMrBNP and in variant genes are present in testis R NAs only during the mid-spermatid stage of spermatogenesis, at the same tim e that HM(r)BNPs in their highly phosphorylated form first appear in the nu cleus. Judging by the lack of a lag between HMrBNP mRNA synthesis and trans lation, the mRNAs for these highly basic proteins are not stored for any le ngth of time. Instead, the deposition of HM(r)BNPs onto DNA, which coincide s with the major reorganization and silencing of the chromatin, may be cont rolled by dephosphorylation.