Comparison of features of the RNase activity of 5'-exonuclease-1 and 5'-exonuclease-2 of Saccharomyces cerevisiae.

Features of the catalytic specificities of 5'-exonuclease-1 (Xrn1) and 5'-exonuclease-2 of Saccharomyces cerevisiae have been compared. For analysis of in vitro properties, 5'-exonuclease-2 has been highly purified, and data show that it is present in yeast cells at about 5-10% of the level of Xrn1. The basic features of the exonuclease activity of the two enzymes, i.e., their activities with RNA and ssDNA and their mode of action, are similar. We have initiated an analysis of structural elements (artificial and natural) that stall the exonucleolytic hydrolysis by the enzymes. A (G)18 artificial sequence in MFA2 mRNA stalls the hydrolysis by both enzymes, yielding a 3' stall fragment. The specific structural element(s) involved is being investigated. To access a similar in vivo specificity of the two enzymes, the effect of overexpression of the essential HKE1 gene encoding exonuclease-2 on several of the phenotypes of xrn1 cells has been examined. The results show that the slow growth rate is partially overcome and that the level of accumulation of specific cytoplasmic RNAs (fragments of the internal transcribed spacer 1 of pre-rRNA and poly (A)-deficient mRNAs) is reduced to 30-40% of the value found in the xrn1 cells.