ENZYMATIC-ACTIVITY OF MELONIN, A TRANSLATIONAL INHIBITOR PRESENT IN DRY SEEDS OF CUCUMIS-MELO L

Melonin, a protein from the dry seeds of Cucumis melo L. that inhibits protein synthesis in cell-free systems, is a ribonuclease (RNase). Me lonin degrades poly(C) but not poly(U) at 55 degrees C and in the pres ence of 7 M urea, melonin degrades both poly(C) and poly(U). Reflectin g this, melonin degrades bonds containing cytidine and, to a lesser ex tent, uridine in non-synthetic RNA substrates. The NH2-terminal amino acid of purified melonin was blocked, but the amino acid sequences of some tryptic peptides indicate that melonin has sequence similarity to cusativin, an RNase isolated from Cucumis safivus L., and to the RNas es from Petunia inflata and Nicotiana alata that determine gametophyti c self-incompatibility.