Variations in the Levels of Chloroplast tRNAs and Aminoacyl-tRNA Synthetases in Senescing Leaves of Phaseolus vulgaris

Abstract

The relative amounts of chloroplast tRNAsL", tRNAGIU, tRNAPh., tRNAsThr, and tRNATYr and of chloroplastic and cytoplasmic aminoacyl-tRNA synthetases were compared in green leaves, yellowing senescing leaves, and N6-benzyladenine-treated senescing leaves from bean (Phaseolus vulgaris, var Contender). Aminoacylation of the tRNAs using Escherichia coli aminoacyl-tRNA synthetases indicated that in senescing leaves the relative amount of chloroplast tRNAP'* was significantly lower than in green leaves. Senescing leaves treated with N6-benzyladenine contained higher levels of this tRNA than untreated senescing leaves. No significant change in the relative amounts of chloroplast tRNASL'", tRNAsThr, and tRNATYr was detected in green, yellow senescing, or N6-benzyladine-treated senescing leaves. Relative levels of chloroplast tRNAs were also estimated by hybridization of tRNAs to DNA blots of gene specific probes. These experiments confirmed the results obtained by aminoacylation and revealed in addition that the relative level of chloroplast tRNAGIU is higher in senescing leaves than in green leaves.Transcription run-on assays indicated that these changes in tRNA levels are likely to be due to a differential rate of degradation rather than to a differential rate of transcription of the tRNA genes. Chloroplastic and cytoplasmic leucyl-, phenylalanyl-, and tyrosyltRNA synthetase activities were greatly reduced in senescing leaves as compared to green leaves, whereas N6-benzyladeninetreated senescing leaves contained higher enzyme activities than untreated senescing leaves. These results suggest that during senescence, as well as during senescence-retardation by cytokinins, changes in enzyme activities, such as aminoacyl-tRNA synthetases, rather than reduced levels of tRNAs, affect the translational capacity of chloroplasts.cies and aminoacyl-tRNA synthetases. In plants, it has been shown that a loss of specific tRNAs occurs in chloroplasts during senescence of leaves (25). Recently, it was shown that in chloroplasts there is a correlation between the amounts of tRNAs specific for a given amino acid and the frequency of the codons specifying this amino acid (16). Furthermore, for the amino acids coded for by more than one codon, the population of isoaccepting tRNAs is adjusted to the frequency of the corresponding synonymous codons. Such an adjustment is probably necessary to ensure maximum efficiency in chloroplast protein biosynthesis (16). Thus, during senescence, limiting quantities of specific chloroplast tRNAs or aminoacyl-tRNA synthetases could restrict the chloroplast's capacity to read certain codons, and the resulting decrease in synthesis of essential cellular protein components would lead to deleterious effects on cell function.It is well known that administration of cytokinins to senescing plants markedly delays leaf senescence and maintains or even increases the levels of RNAs and proteins (1 1, 12, 15, 24). In particular, it has been suggested that cytokinin treatment of plants at different stages o...

Chloroplast tRNAAsp: nucleotide sequence and variation of in vivo levels during plastid maturation

“…Although transcription does not seem to play a major role in the regulation of gene expression in plastids, the levels of individual tRNAs can differ during different stages of plant development: the level of tR-NA alu in plastids from senescing bean (Phaseolus vulgaris) leaves increases relative to the levels of tRNA I~, tRNA Tyr and tRNA TM. In these cases, the amounts of individual tRNAs seem to be regulated by their different half-lives in organello rather than by different transcription rates [ 12]. Although the present study does not allow any conclusions about the kinetics of tRNA expression, these results corroborate our finding that the relative levels of tRNAs encoded on even a single chloroplast transcription unit (the trnE-trnYtrnD operon) can change during plant development.…”

Leaf senescence and activities of the antioxidant enzymes

“…Transfer RNA synthetase activities are greatly reduced during senescence (Jaybaskaran et al 1990). A general increase in RNase activities has been described during senescence (Green 1994, Buchanan-Wollaston 1997.…”

“…For example a notable increase in the activity of peroxisomal MnSOD and a decline in the activity of mitochondrial MnSOD from pea leaf extract were determined at the same time (Pastori and del Río 1994). On the other hand a decrease in the activity of whole pea leaf extract MnSOD was described in another experiment (Jimenéz et al 1998). Moreover differences between natural and artificially induced senescence would deserve a separate article.…”

Cytokinins in Tobacco and Wheat Chloroplasts. Occurrence and Changes Due to Light/Dark Treatment

“…This control may be mediated by polyribosome formation (Ohya and Suzuki, 1998), modification of the secondary structure of poly(A ϩ ) RNA (Jackowski et al, 1987), phosphorylation of the ribosomal proteins (Yakovleva et al, 1992), or regulation of the activity of tRNAs (Romanov, 1990). CKs (specifically, benzylaminopurine [BAP]) have also been reported to increase the activity of aminoacyltRNA synthetases in senescing bean leaves (Jayabaskaran et al, 1990). A correlation between two CK-regulated processes, cell division and chloroplast biogenesis, has been reported (Reski and Abel, 1992;Suzuki et al, 1992).…”