Figure 6

Model of Piwi-interacting RNA (piRNA) cluster formation. (A) Proto-piRNA cluster: transcripts are produced from a proto-piRNA-producing locus. (B) Conversion to piRNA-producing locus: a specific transcription factor, histone marker, DNA methylation pattern, and/or RNA-binding protein (blue arrow, circle, and oval, respectively) convert the proto-piRNA-producing locus into a piRNA-producing site. (C) Sequential transposition event: the open nature of chromatin at the piRNA-producing locus attracts transposon integration (left panel). Certain types of transposons can accept the transposition within themselves (right panel). (D) Maturation of piRNA cluster: a mature piRNA cluster is produced through sequential transposition events at piRNA-producing loci.