A unique nucleosome arrangement, maintained actively by chromatin remodelers facilitates transcription of yeast tRNA genes

Abstract Insertions of the yeast element Ty3 resulting from induced retrotransposition were characterized in order to identify the genomic targets of transposition. The DNA sequences of the junctions between Ty3 and flanking DNA were determined for two insertions of an unmarked element. Each insertion was at position -17 from the 5' end of a tRNA-coding sequence. Ninety-one independent insertions of a marked Ty3 element were studied by Southern blot analysis. Pairs of independent insertions into seven genomic loci accounted for 14 of these insertions. The DNA sequence flanking the insertion site was determined for at least one member of each pair of integrated elements. In each case, insertion was at position -16 or -17 relative to the 5' end of one of seven different tRNA genes. This proportion of genomic loci used twice for Ty3 integration is consistent with that predicted by a Poisson distribution for a number of genomic targets roughly equivalent to the estimated number of yeast tRNA genes. In addition, insertions upstream of the same tRNA gene in one case were at different positions, but in all cases were in the same orientation. Thus, genomic insertions of Ty3 in a particular orientation are apparently specified by the target, while the actual position of the insertion relative to the tRNA-coding sequence can vary slightly.

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Cleavage specificity of chloroplast and nuclear tRNA 3'-processing nucleases.

Molecular and Cellular Biology ◽

10.1128/mcb.12.2.865 ◽

1992 ◽

Vol 12 (2) ◽

pp. 865-875 ◽

Cited By ~ 26

Author(s):

A Oommen ◽

X Q Li ◽

P Gegenheimer

Keyword(s):

Substrate Specificity ◽

Processing System ◽

Trna Genes ◽

Wheat Embryo ◽

Enzyme Preparations ◽

Yeast Trna ◽

Cleavage Specificity ◽

Mature Transcript ◽

Enzyme Fraction ◽

Chloroplast Trna Genes

tRNAs in eukaryotic nuclei and organelles are synthesized as precursors lacking the 3'-terminal CCA sequence and possessing 5' (leader) and 3' (trailer) extensions. Nucleolytic cleavage of the 3' trailer and addition of CCA are therefore required for formation of functional tRNA 3' termini. Many chloroplast tRNA genes encode a C at position 74 which is not removed during processing but which can be incorporated as the first base of the CCAOH terminus. Sequences downstream of nucleotide 74, however, are always removed. Synthetic yeast pre-tRNA(Phe) substrates containing the complete CCA74-76 sequence were processed with crude or partially purified chloroplast enzyme fractions. The 3'-extended substrates (tRNA-CCA-trailer) were cleaved exclusively between nucleotides 74 and 75 to give tRNA-COH, whereas a 3'-mature transcript (tRNA-CCAOH) was not cleaved at all. A 5'-, 3'-extended chloroplast tRNA-CAG-trailer was also processed entirely to tRNA-COH. Furthermore, a 5'-mature, 3'-extended yeast pre-tRNA(Phe) derivative, tRNA-ACA-trailer, in which C74 was replaced by A, was cleaved precisely after A74. In contrast, we found that a partially purified enzyme fraction (a nuclear/cytoplasmic activity) from wheat embryo cleaved the 3'-extended yeast tRNA(Phe) precursors between nucleotides 73 and 74 to give tRNA(OH). This specificity is consistent with that of all previously characterized nuclear enzyme preparations. We conclude that (i) chloroplast tRNA 3'-processing endonuclease cleaves after nucleotide 74 regardless of the nature of the surrounding sequences; (ii) this specificity differs from that of the plant nuclear/cytoplasmic processing nuclease, which cleaves after base 73; and (iii) since 3'-mature tRNA is not a substrate for either activity, these 3' nucleases must require substrates possessing a 3'-terminal extension that extends past nucleotide 76. This substrate specificity may prevent mature tRNA from counterproductive cleavage by the 3' processing system.

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