Properties of the exonuclease activity that degrades H4 histone mRNA

We have described a cell-free system for studying mRNA degradation (Ross, J., and Kobs, G. (1986) J. Mol. Biol. 188, 579-593). Using that system we found that human H4 histone mRNA was degraded in a 3' to 5' direction by an exonucleolytic activity. Here we investigate several properties of the crude system and of the exonuclease. A RNase inhibitor, such as that from placenta, was required to block nonspecific ribonucleases and thereby to permit different mRNAs to be degraded at different rates. The histone mRNA exonuclease required divalent cation (magnesium) but not exogenously added ATP or GTP. It functioned efficiently at monovalent cation concentrations ranging from 0.5 to 200 mM. It was bound to ribosomes isolated from cells lysed in low salt buffers. However, it was eluted in active form from the ribosomes by exposing them to 0.3 M KCl. The enzyme rapidly degraded deproteinized, 32P-labeled histone mRNA prepared enzymatically.