No abstract

The effect of cadmium on growth of Cd-tolerant and -sensitive plants of Silene vulgaris and on the production of metal-binding compounds in both types of plants was studied. The Cd-content of the roots and the Cd-root/shoot ratio was higher in Cd-tolerant plants. A Cd-binding compound (Cd-BC) with an apparent molecular mass of 14.5 kD was isolated from the roots of Cd-tolerant and -sensitive plants, grown in 40 mmol m"^^ Cd for 21 d. More than 60% of the total Cd in the roots was associated with this compound. Determination of the aminoacid content of the purified Cd-containing compound from both types of plants showed that they possessed a similar amino-acid composition to that of phytochelatins. Only the bis-and tris-forms were present. The amount of Cd and sulphide associated with phytochelatin was greater in tolerant plants than in sensitive ones suggesting that an increased sulphide content of complexes containing peptide, sulphide and Cd may form the basis of evolved Cd-tolerance in Silene vulgaris.

Previous phylogenetic analysis of rRNA sequences for covariant base changes has identified -20 potential tertiary interactions. One ofthese is present in domain III ofthe large subunit rRNA and consists of two adjacent Watson-Crick base pairs that, in Saccharomyces cerevisiae 26S rRNA, connect positions 1523 and 1524 to positions 1611 and 1612. This interaction would strongly affect the structure of an evolutionarily highly conserved region that acts as the binding site for the early-assembling ribosomal proteins L25 and EL23 of S. cerevisiae and Escherichia coli, respectively. To assess the functional importance of this tertiary interaction, we determined the ability of synthetically prepared S. cerevisiae ribosomal protein L25 to associate in vitro with synthetic 26S rRNA fragments containing sequence variations at positions 1523 and 1524 and/or positions 1611 and 1612. Mutations that prevent the formation of both base pairs abolished L25 binding completely, whereas the introduction of compensatory mutations fully restored protein binding. Disruption of only the U1524AA611 pair reduced L25 binding to -30% of the value shown by the wild-type 26S rRNA fragment, whereas disruption of the G5C23-C"612 base pair resulted in almost complete loss of protein binding. These results strongly support the existence and functional importance of the proposed doublet tertiary interaction in domain Ill of the large subunit rRNA.Ribosomes are highly complex ribonucleoprotein particles that catalyze the sequential linkage of amino acids in the order specified by the codon sequence of each particular mRNA. Initially, the biological activity of the ribosome was thought to reside in the protein moiety. Over the past years, however, a considerable body of evidence has been collected in support of the view (1) that the rRNAs, rather than merely ensuring the correct spatial arrangement of the ribosomal proteins, participate directly in ribosomal function (for a recent review, see ref.2) and might even be the fundamental functional determinants of the ribosome (3, 4). Regions crucial for each of the three main phases of translation have been identified in both the small and large subunit rRNA, predominantly by in vivo mutational analysis of the Escherichia coli rRNA species and by studies on the nature of various antibiotic-resistant mutations (for reviews, see refs. 2, 5, and 6). The strong evolutionary conservation of the primary and/or secondary structure of most of these regions supports their importance for the pertinent biological functions in all types of ribosomes.Phylogenetic comparison of the large number of rRNA sequences presently available has provided us with a fairly detailed picture of the secondary structure of the different types of rRNA (7,8). Our knowledge of the three-dimensional folding of these molecules, which is of paramount importance for a full understanding of their function, is still very limited, however. Relatively rough models of the conformation of E. coli 16S and 23S rRNA have been developed (9...

In yeast, the 39 end of mature 18S rRNA is generated by endonucleolytic cleavage of the 20S precursor at site D. Available data indicate that the major cis-acting elements required for this processing step are located in relatively close proximity to the cleavage site. To identify these elements, we have studied the effect of mutations in the mature

The nucleotide sequence of 6472 base pairs of an 8.2 kb segment of Saccharomyces cerevisiae chromosome XI has been determined. The sequence contains a cluster of four long open reading frames (ORF) designated YKL2, YKL3, YKL4 and TGL1 in the same orientation, flanked at the 5'-end by a divergent incomplete ORF (YKL1). Transcription and Southern analysis of the four complete ORFs showed that all are expressed and are present in single copy on the haploid genome. The average codon adaptation index of the coding regions is approximately 0.2, suggesting that these genes are lowly expressed. The upstream regions of all four genes as well as the YKL1 ORF contain putative promoter elements previously found to be characteristic of nuclear genes encoding mitochondrial proteins. Significant sequence similarities were found between the YKL3 protein and Escherichia coli ribosomal protein S2 as well as between the TGL1 protein and triglyceride lipases from rat salivary gland and human gastric tissue. The 3'-end of the 6472 bp nucleotide sequence overlaps with the upstream region of the previously identified CTK1 gene, encoding the largest subunit of CTD kinase (Lee, J.M. and Greenleaf, A.L., 1991, Gene Expression 2, 149-167), thereby increasing the number of genes on the 8.2 kb fragment to at least five. The transcripts of these genes represent approximately 83% of the DNA fragment, making it one of the most highly transcribed regions of the yeast chromosome analysed to date.