CDNA STRUCTURE AND REGULATORY PROPERTIES OF A FAMILY OF STARVATION-INDUCED RIBONUCLEASES FROM TOMATO

In previous work we have determined the primary structure of two of th e five ribonucleases which are induced by phosphate starvation in cult ured tomato cells. Here, we present the isolation and characterization of the cDNAs for the extracellular ribonuclease LE and the intracellu lar, but extravacuolar ribonuclease LX. Structural analysis of these c DNAs together with partial protein-sequencing of vacuolar ribonuclease s LV1, LV2 and LV3 revealed a family of very similar ribonucleases. Fr om these data we assume identity between ribonucleases LE and LV3 for which the targeting mechanism has to be shown. Furthermore, RNase LV1 and RNase LV2 might be posttranslational processing products of RNase LX which travel to the vacuoles after splitting off the putative ER re tention signal present at RNase LX. Additionally, we show by northern blot analysis that phosphate starvation in plant cells leads to an inc rease in the steady-state level of this type of enzymes revealing clos e similarities of the plant response to a limited supply of inorganic phosphate with the PHO regulation in bacteria and fungi.