Euglena gracilis chloroplast transfer RNA transcription units. I. Physical map of the transfer RNA gene loci

Fractionation and identification of Euglena gracilis cytoplasmic and chloroplastic tRNAs and mapping of tRNA genes on chloroplast DNA

The cytoplasmic and chloroplast tRNAs of Euglena gracilis Z strain were fractionated by two-dimensional gel electrophoresis and identified by aminoacylation. Purified chloroplast tRNAs, labeled in vitro with |(32)P|, were hybridized to endonuclease restriction fragments of chloroplast DNA, allowing the corresponding tRNA genes to be localized on the physical map of Euglena chloroplast DNA.

Localization of DNA in the condensed interphase chromosomes of Euglena

The localization of DNA in the condensed interphase chromosomes of Euglena was determined by immunoelectron microscopy. Deposits of gold particles that coincided with the localization of DNA followed threads that corresponded to the chromatin fibers. The threads were 55-80nm in diameter and were assumed to be supersolenoids. The localization of gold deposits on chromosomes that had been sectioned in various directions suggested that the chromatin fibers coiled around the surface of chromosomes, with a wide central axial region of the chromosomes remaining free of DNA. These findings are discussed in relation to current models of chromosomal structure.

Intercistronic group III introns in polycistronic ribosomal protein operons of chloroplasts

A novel ribosomal protein operon in the Euglena gracilis chloroplast genome was characterized. It encodes the genes for ribosomal proteins S4 and S11 (rps4 and rps11). The coding region of the rps11 gene is interrupted by two introns of 107 and 100 bp. The introns belong to a distinct class known as group III introns. The major transcript from this operon was characterized as a fully spliced dicistronic rps4-rps11 mRNA by RNA blot analysis, primer extension sequencing, and cDNA cloning and sequencing. An additional 95 nucleotide (nt) group III intron was identified in the 123 nt rps4-rps11 intercistronic region. The identification of the intercistronic intron between the rps4 and rps11 genes was unexpected. Other RNA transcripts from regions of the genome that could potentially contain intercistronic introns were re-examined and two other intercistronic, group III introns were found. These are located in a large ribosomal protein operon between the genes for the ribosomal proteins L23 and L2, and between L14 and L5. There are at least 50 group III introns in the E. gracilis chloroplast genome. All but 6 are found in genes encoding protein components of the transcriptional and translational apparatus. The distribution of group III introns and the unusual location of intercistronic group III introns may reflect some aspect of gene expression, or provide some insight into the mechanism of their splicing.

Characterization of the TrnD, TrnK, PsaA locus of Euglena gracilis chloroplast DNA

The EcoRI fragment Eco J' of Euglena gracilis chloroplast DNA has previously been identified as a tRNA coding locus. The nucleotide sequence of a 2.3-kb region of the Eco J' fragment known to contain the tRNA genes has been determined. This locus was found to contain two tRNA genes, trnD-GTC and trnK-TTT. Separated from the trnK locus by a 43-bp spacer is an open reading frame of 398 codons. The open reading frame is 73-75% homologous to the amino-terminal coding regions of the spinach and maize genes for the P700 chlorophyll a apoprotein of photosystem I. It has been identified as exon I of an intron-containing psaA gene. The exon is followed by an intron of at least 214 bp that has features characteristic of other Euglena chloroplast introns. Major chloroplast RNA transcripts of sizes 5.5, 4.7, and 2.3 kb hybridize to a psaA-specific probe. The gene for a second photosystem I P700 apoprotein, psaB, has been located on an adjacent EcoRI fragment, Eco C.

Transfer RNA genes of Euglena gracilis chloroplast DNA : A review

Transfer RNA genes have been mapped to at least nine different loci on the physical map of the Euglena gracilis chloroplast genome. One of these loci in the ribosomal RNA operons is present three times per genome. The DNA sequences of six of the nine different loci, containing 21 different tRNA genes, have been determined. Genes corresponding to the amino acids Ala, Arg, Asn, Cys, Gln, Gly (2), Glu, His, Ile, Leu (2), Met (2), Phe, Ser, Thr, Trp, Tyr, Val, and one unassigned species have been identified. All genes except one are found in clusters of 2-6 genes. None of the known genes contains introns, nor codes for the 3'-CCA terminus. In addition to these genes, two pseudo tRNA genes are present in the rDNA leader region.

Selective in vitro transcription of chloroplast genes

Transcription of Euglena gracilis chloroplast genes has been investigated by using in vitro transcription systems. A DNA-dependent RNA polymerase responsible for the transcription of rRNA genes has been isolated as a nucleoprotein complex (transcriptionally active chromosome). The RNA polymerase remains tightly bound to the chloroplast DNA template and does not initiate transcription with cloned chloroplast genes. A transcriptionally active extract has been prepared from intact Euglena chloroplasts. The soluble RNA polymerase in this extract recognizes cloned chloroplast tRNA genes and tRNA-sized products have been detected after transcription. The tRNA-sized molecules specifically hybridize to the tRNA genes in the plasmid DNA. At least five tRNA-sized products have been identified from transcription of a trnY1trnH1-trnM1-trnE1-trnW1-trnG1 cluster. Evidence is also presented that processing enzymes in the chloroplast-extract can recognize a polycistronic tRNAVal-tRNAAsn-tRNAArg precursor and process it into tRNA-sized molecules. Truncated templates have been used to demonstrate that the chloroplast tRNA genes are actively transcribed. From a comparison of 5' flanking sequences in chloroplast tRNA genes, a consensus sequence which might function as a promoter, has been identified. The properties of the RNA polymerase involved in the transcription of chloroplast rRNA genes and tRNA genes have been investigated and compared.