Shimizu T, Inoue T, Shiraishi H
Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Gene. 2001 Aug 22;274(1-2):227-35. doi: 10.1016/s0378-1119(01)00601-1.
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.
体细胞和生殖细胞。体细胞是终末分化的、有丝分裂后的细胞，每一代都会经历逐渐衰老直至细胞死亡。为了阐明与衰老相关的大分子自我降解过程，我们试图克隆一种在衰老过程中mRNA积累增加的核糖核酸酶。相应的cDNA克隆VRN1编码卡特氏团藻的一种S类核糖核酸酶，是第一个从绿藻中克隆出来的T(2)/S类核糖核酸酶。半定量RT-PCR分析表明，以核糖体蛋白S18的mRNA作为内参时，衰老体细胞中VRN1 mRNA的相对含量比幼体细胞高3倍以上。VRN1 mRNA不会被磷酸盐饥饿诱导，这表明其在衰老过程中的积累不是由于利用磷酸盐时自身诱导的缺陷所致。对于RNS3也有类似的调控报道，RNS3编码拟南芥衰老叶片中诱导产生的S类核糖核酸酶。这些观察结果表明，VRN1可能在卡特氏团藻体细胞衰老过程中促进RNA降解，并且其调控与高等植物中某些衰老相关核糖核酸酶的调控相似。
