Histone chaperone that stabilizes pre-existing histone tetramers and regulates replication-independent histone exchange on chromatin (PubMed:26109053). Required for normal chromatin refolding in the coding region of transcribed genes, and for the suppression of spurious transcription (PubMed:26109053). Global regulatory protein that plays positive as well as negative regulatory roles in transcription (PubMed:2072912). Interacts with tetramers formed by histone H3 and H4 (PubMed:26109053). Interacts with SAP1 and CDC23 (PubMed:8654588, PubMed:8710860). Component of the SAGA complex, a large multiprotein complex that modifies the chromatin, at least composed of SPT2, SPT7, SPT8, SPT20/ADA5, HFI1/ADA1, ADA2, ADA3/NGG1, TRA1 and GCN5 (PubMed:9885573). The acidic C-terminal domain mediates interaction with histone H3/H4 complexes. Present with 1240 molecules/cell in log phase SD medium. Belongs to the SPT2 family.