Sperm chromatin decondensation by template activating factor I through direct interaction with basic proteins

Template activating factor I (TAF-I) was originally identified as a host factor required for DNA replication and transcription of adenovirus genome complexed with viral basic proteins. Purified TAF-I was shown to bind to core histories and stimulate transcription from nucleosomal templates. Human TAF-I consists of two acidic proteins, TAF-Iα and TAF-Iβ, which differ from each other only in their amino-terminal regions. Here, we report that TAF-I decondenses demembraned Xenopus sperm chromatin. Human TAF-Iβ has a chromatin decondensation activity comparable to that of NAP-I, another histone binding protein, whereas TAF-Iα has only a weak activity. Analysis of molecular mechanisms underlying the chromatin decondensation by TAF-I revealed that TAF-I interacts directly with sperm basic proteins. Deletion of the TAF-I carboxyl-terminal acidic region abolishes the decondensation activity. Interestingly, the acidic region itself is not sufficient for decondensation, since an amino acid substitution mutant in the dimerization domain of TAF-I which has the intact acidic region does not support chromatin decondensation. We detected the β form of TAF-I in Xenopus oocytes and eggs by immunoblotting, and the cloning of its cDNA led us to conclude that Xenopus TAF-Iβ also decondenses sperm chromatin. These results suggest that TAF-I plays a role in remodeling higher-order chromatin structure as well as nucleosomal structure through direct interaction with chromatin basic proteins.