Figure 4.

Structural insights into Sxl function. (A) Sxl bound to the proximal pyrimidine tract of tra mRNA [82; PDB: 1B7F]. The two RRMs of Sxl (gray) specifically recognize the U‐rich pyrimidine tract of tra mRNA (green). RNA contacts are mostly established with the beta‐sheet surfaces of the two RRMs. (B) Structure of the Sxl‐Unr repressor complex bound to a fragment of msl‐2 mRNA [83; PDB: 4QQB]. Binding of Sxl (gray) to the 3′ UTR regulatory elements of msl‐2 mRNA (green) occurs in a manner comparable to recognition of tra mRNA. Upon binding of Unr (cold shock domain 1, shown in orange), additional, ternary contacts are formed that involve the side of RRM1 of Sxl and allow specific recognition of an extended stretch of the RNA.