sentence: In Drosophila melanogaster , the degradation of maternal mRNA is regulated by the activity of the Pumilio , Smaug proteins and miRNAs in the developing embryo [ 25,26,27 ] , while the initial activation of zygotic gene transcription is controlled by the zinc-finger protein Zelda ( zelda , zld ) which binds to TAG team sites in the promoter regions of downstream genes , activating their expression [ 28,29 ] .