OAT PHYTOCHROME-A MESSENGER-RNA DEGRADATION APPEARS TO OCCUR VIA 2 DISTINCT PATHWAYS

We have identified possible mechanisms for the degradation of oat phyt ochrome A (PHYA) mRNA. The majority of PHYA mRNA molecules appeared to be degraded prior to removal of the poly(A) tail, a pathway that diff ers from that reported for the degradation of other eukaryotic mRNAs. Polyadenylated PHYA mRNA contained a pattern of putative degradation p roducts that is consistent with a 5'-->3' exoribonuclease, although th e participation of a stochastic endoribonuclease cannot be excluded. T he poly(A) tail of PHYA mRNA was heterogeneous in size and ranged from similar to 14 to 220 nucleotides. Early PHYA mRNA degradation events did not appear to involve site-specific endoribonucleases. Approximate ly 25% of the apparently full length PHYA mRNA was poly(A) deficient. Oat H4 histone, beta-tubulin, and actin mRNA populations had lower amo unts of apparently full-length mRNAs that were poly(A) deficient. Degr adation of the poly(A)-deficient PHYA mRNA, a second pathway, appeared to be initiated by a 3'-->5' exoribonucleolytic removal of the poly(A ) tail followed by both 5'-->3' and 3'-->5' exoribonuclease activities . Polysome-associated RNA contained putative PHYA mRNA degradation pro ducts and was a mixture of polyadenylated and deadenylated PHYA messag es, suggesting that the two distinct degradation pathways are polysome associated.