Polysomal ribonuclease 1 exists in a latent form on polysomes prior to estrogen activation of mRNA decoy

Estrogen induces a global change in the translation profile of Xenopus hepa tocytes, replacing serum protein synthesis with production of the yolk prot ein precursor vitellogenin. This is accomplished by the coordinate destabil ization of serum protein mRNAs and the transcriptional induction and subseq uent stabilization of vitellogenin mRNA. Previous work identified an endonu clease activity whose appearance on polysomes correlated with the disappear ance of serum protein mRNAs, This enzyme, polysomal ribonuclease 1 (PMR1), is a novel member of the peroxidase gene family, The current study examined the association of PMR1 with its mRNA targets on polysomes and mRNPs. The highest amount of polysome-bound PMR1 was observed prior to estrogen induct ion of mRNA decay, Its distribution on sucrose density gradients matched th e absorbance profile of polysome-bound mRNA, suggesting that PMR1 forms a l atent complex with mRNA, Following dissociation with EDTA the 62 kDa PMR1 s edimented with a larger complex of >670 kDa. Estrogen induces a 22-fold inc rease in unit enzymatic activity of polysome-bound PMR1, and a time-depende nt loss of PMR1 from polysomes in a manner that mirrors the disappearance o f albumin mRNA, These data suggest that the key step in the extensive estro gen-induced change in mRNA decay in Xenopus liver is activation of a latent mRNA endonuclease associated with its target mRNA.