Histone chaperone that stabilizes pre-existing histone tetramers and regulates replication-independent histone exchange on chromatin. Required for normal chromatin refolding in the coding region of transcribed genes, and for the suppression of spurious transcription. Global regulatory protein that plays positive as well as negative regulatory roles in transcription. Interacts with tetramers formed by histone H3 and H4. The acidic C-terminal domain mediates interaction with histone H3/H4 complexes. Belongs to the SPT2 family.