A senescence-associated S-like RNase in the multicellular green alga Volvox carteri

Asexual individuals of the green alga Volvox carteri consist of only two cell types: somatic and reproductive cells. The somatic cells are terminally differentiated, post-mitotic cells which undergo gradual senescence leading to cell death in every generation. To elucidate the self-degrading process of macromolecules associated with senescence, we attempted to clone an RNase whose mRNA accumulation is increased during senescence. The corresponding cDNA clone VRN1, encoding an S-like RNase of V. carteri, is the first T(2)/S-like RNase to be cloned from green algae. Semi-quantitative RT-PCR analysis revealed that a relative amount of VRN1 mRNA is more than three-fold higher in the senescent somatic cells than in young somatic cells when the mRNA of ribosomal protein S18 is used as an internal standard. VRN1 mRNA is not induced by phosphate starvation, indicating that its accumulation during senescence is not due to a self-induced defect in utilizing phosphates. Similar regulation has been reported for RNS3, which encodes the S-like RNase that is induced in senescent leaves of Arabidopsis thaliana. These observations imply that VRN1 may promote RNA degradation during senescence of somatic cells in V. carteri, and that its regulation has similarity with that of certain senescence-associated RNases in higher plants.