Expression of DjY1, a protein containing a cold shock domain and RG repeat motifs, is targeted to sites of regeneration in planarians.

Planarians are well-known for their exceptional regenerative abilities. This investigation focuses on the involvement of a Y-box protein, defined by the presence of a cold-shock domain, in regeneration-specific processes. Previous studies have shown that developmentally expressed Y-box proteins bind to mRNA molecules and regulate the timing of their translation. We have isolated and characterized a planarian Y-box gene, DjY1, which is specifically expressed at the site of regeneration, the blastema. DjY1 transcripts appear rapidly at the site of cutting and increase in number as the blastema grows. The timing and level of expression is similar irrespective of the orientation of the cut: in anterior, posterior, and lateral regenerative tissue. As regeneration nears completion, there is a general decrease in transcript level except in structures which are still differentiating, specifically in the auricles where new DjY1 transcripts are produced. A similarly modulated temporal pattern of expression throughout regeneration is seen in assaying the DjY1 protein. Within the population of blastemal cells, a subset of differentiating cells is specifically immunostained using antibodies to DjY1. The DjY1 protein contains a cold-shock domain and RG-repeat motifs, both of which are associated with RNA-binding properties: in vitro binding studies using recombinant DjY1 show that the preferred template is single-stranded RNA of heterogeneous sequence. These data provide the first direct evidence that a Y-box protein is involved in the regeneration process in planarians and implicate DjY1 in the translational regulation of differentiation-specific mRNAs.