Transcription elongation factor that enhances the transcription elongation by RNA polymerase II (RNAPII) (By similarity). Plays an important role in regulating embryo apical and basal patterning during early embryogenesis, partly through negative regulation of the transcription factors PHABULOSA and PHAVOLUTA. Interacts (via N-terminus) with IWS1. Expressed in shoot apical meristem, leaf primordia, vasculature of young leaves, inflorescence meristem, floral meristem, young floral organs, developing ovules and anthers. During embryo development, expressed throughout embryos at the globular, heart, torpedo and bent-cotyledon stages. Embryonic lethality leading to aborted seed development. A number of isoforms are produced. According to EST sequences. Belongs to the SPT6 family.