Complex sentence: Two single MYB transcription factors, CCA1 and LHY, activate PRR7 and PRR 9, which then repress CCA1 and LHY through the binding of their promoters.
Split into simpler sentences: Two single MYB transcription factors, CCA1 and LHY, activate PRR7 and PRR 9. In turn, PRR7 and PRR9 repress CCA1 and LHY through the binding of their promoters.