The genetic fine structure of nonsense suppressors in Schizosaccharomyces pombe

Summary Meiotic and mitotic fine-structure maps of two efficient UGA suppressors of Schizosaccharomyces pombe which are known (sup3-e) or inferred (sup9-e) to code for two serine tRNAs carrying the mutant anticodon U*CA (Kohli et al. 1979a, b, Rafalski et al. 1979) are presented. Maps based on spontaneous meiotic, spontaneous mitotic and MMS induced mitotic recombination between the primary site of the anticodon mutation and a number of inactivating second-site mutations are similar. Specific marker effects, which drastically increase the frequency of spontaneous meiotic and mitotic recombination in crosses involving one or the other of four exceptional sites (including the anticodon sites of both sup3-e and sup9-e), disappear when mapping is based on MMS induced mitotic recombination. The meiotic marker effect characterizing the anticodon site of one of the two efficient UGA suppressors (sup3-e) also disappears upon further mutation to an inefficient UAA suppressor allele (sup3-i), as shown by its absence in a fine-structure map based on meiotic recombination between the anticodon mutation of this ochre suppressor allele and a new set of inactivating second-site mutations derived from it.     

Gene conversion in nonsense suppressors of Schizosaccharomyces pombe

Summary Gene conversion and postmeiotic segregation patterns have been analysed at 14 mutant sites of sup3, sup8 and sup9 including 5 alleles with a strong marker effect on recombination frequencies in two-factor crosses. The total frequency of gene conversion and postmeiotic segregation tetrads is fairly constant within each gene, but may vary from one gene to another. About 97% of the conversion events are coconversions spanning the whole sup gene. Postmeiotic segregations are usually quite rare. None of the marker-effect alleles has an increased rate of hybrid DNA formation at the allele considered, as judged from the frequency of gene conversion and postmeiotic segregation in one-factor crosses. At least two of them, sup3-e and sup9-e, are associated with a high frequency of postmeiotic segregation indicating a poor repair of the corresponding base-pair mismatches. This is also observed in a two-factor cross and can account for the marker effect on recombination frequencies. The properties of a third marker effect allele, sup3-e,r10, are best explained by a higher probability of single site conversions as opposed to coconversions in two-factor crosses involving the mutant site r10.     

A revised chromosome map of the fission yeast Schizosaccharomyces pombe

Summary The genetic map of the nuclear genome of the fission yeast Schizosaccharomyces pombe has been extended by mitotic and meiotic mapping data. A total of 158 markers are now assigned to the three linkage groups known in this organism, and 118 of them have been located on the corresponding chromosome map. Chromosome II and III each consist of one linkage group. There is some indication that the two large fragments which define chromosome I are meiotically linked, but the linkage observed is significant at the P = 0.05 level only. The length of the map is at least 1,700 map units, corresponding to an average of about 8 kilobases per map unit. The latter figure is comparable to the one obtained for intragenic recombination in the sup3 gene (Hofer et al. 1979). The basic frequency of gene conversion as measured for 21 genes varies according to a distribution of Poisson (with a modal value of 0.6% conversion per meiosis and per gene), in sharp contrast with Saccharomyces cerevisiae (Fogel et al. 1980) and Ascobolus immersus (Nicolas 1979). This may reflect the rarity of gene or region-specific rec alleles in S. pombe and may be related to the homothallism of this organism.     

Two transfer RNA sequences abut the large ribosomal RNA gene in Tetrahymena mitochondrial DNA: tRNAleu (anticodon UAA) and tRNAmet (anticodon CAU)

Summary The sequence of a 1,427 base pair restriction fragment, HaeIII fragment 6, of the ciliate protozoan Tetrahymena mitochondrial DNA, is presented. The first 780 nucleotide sequence aligns well with the terminal segment of the large rDNA sequence of Paramecium mitochondria. Immediately abutting this rDNA termination sequence, a tRNA sequence was found with anticodon UAA for leucine. The derived tRNA sequence is 81 bases long without the 3 CCA end, has a high G+C content of 48.1%, and can be folded into a normal cloverleaf structure with mostly conserved bases and normal stems and loops. The tRNA sequence found at an analogous position of the Paramecium mitochondrial DNA is tRNA^tyr. Following a highly A+T rich sequence of 300 base pairs, another tRNA-like sequence is present; this putative tRNA has only 67 bases with anticodon CAT (Met) and forms standard aminoacyl, anticodon and TC stems with a conventional TC loop. However, the DHU loop and stem are unusually short and irregular; the base at position 8 is G instead of T; and the base following the anticodon, which is normally a purine, is T. The significance of these tRNA structures is discussed.     

The DEL1 mutator gene in Saccharomyces cerevisiae does not act in trans

Summary DEL1 strains of the yeast Saccharomyces cerevisiae exhibit a high rate of deletions of the three linked genes, CYC1, OSM1, and RAD7. Classical genetic methods showed that DELI segregated as a single Mendelian gene closely linked to CYC1. In addition, genetic evidence suggested that DEL1 was both cis- and trans-dominant (Liebman et al. 1979). Molecular analysis of deletions isolated from a haploid DEL1 strain established that deletion formation was mediated by recombination between yeast transposable elements, Ty's (Liebman et al. 1981). We now report the molecular characterization of deletions isolated from diploids in the trans configuration. This analysis reveals that these deletions probably arose in a two-step process involving mitotic recombination followed by Ty-mediated deletion formation in cis.     

Mitotic recombination in stable and unstable chromosome III disomics

Summary Approximately 2% of the haploid breakdown sectors of heterozygous chromosome III disomics of Aspergillus nidulans are the result of recombination between the homologous chromosomes. The exchanges are concentrated between the two mutations spanning the centromere. Comparisons are made between disomics hemizygous for the sod ^III A1 mutation (Upshall et al. 1979) which are stable when grown at 37 °C, and disomics carrying the wild type allele of the sod ^IIIA1 locus, which are unstable under all conditions. It is shown that neither temperature nor the sod ^IIIA1 mutation affect the frequency or pattern of recombination between the homologues.     

FLP-FRT mediated intrachromosomal recombination on a tandemly duplicated YEp integrant at the ILV2 locus of chromosome XIII in Saccharomyces cerevisiae

Summary A YEp chimaeric plasmid carrying SMR1 and URA3 genetic markers was integrated into chromosome XIII at the ilv2-1 locus in a [cir°] background. The 1.5 kb BglII deletion of ilv2-1 allowed the clear identification of an integrant structure which consisted of a direct tandem duplication (TD) of the chimaeric plasmid. Within the integrant structure, a single copy of the plasmid sequence was flanked by a direct duplication of the 2m site-specific recombinase (FLP) recognition target (FRT). Isogenic [cir°] and [cir ⁺] diploids formed by crossing the [cir°] TD strain to complementary haploids were analyzed for plasmid marker loss and chromosomal DNA alterations in the presence and absence of selection pressure for the URA3 and SMR1 plasmid borne markers. [cir°] diploids showed no plasmid marker loss and maintained the TD structure. In the absence of selection pressure, the [cir ⁺] diploid underwent FLP-FRT mediated unequal interchromatid recombination, resulting in the breakage-fusion-bridge cycle and homozygotization of chromosome XIII (Rank et al. 1988). Maintenance of selection pressure for the centromere distal plasmid URA3 marker selected against FLP-FRT interchromatid recombinants so that the effects of site specific recombinase on intrachromatid recombination could be evaluated. Intrachromatid recombination at the directly duplicated FRT sites of the TD structure resulted in the loss of a diagnostic internal fragment. These results show that in the presence of FLP, FRT sites separated by up to 13.3 kb of chromosomal DNA function as substrates for intra and interchromatid recombination.     

The importance of an intact sympathetic innervation for the differentiation of the β-adrenoceptor subtypes in the rat parotid gland

Acinar cells of the rat parotid gland are in intimate contact with adrenergic nerves, arousing from the ganglion cervicale superior (Bloom et al. 1977). The sympathetic nerves or circulating catecholamines exert their effects by activating α-adrenoceptors (electrolyte and water secretion) or β-adrenoceptors (enzyme secretion) (Batzri et al. 1971, Carlsöö et al. 1981). It is now well established that the β-adrenoceptors can be divided in two subclasses, β₁ and β₂, present in different proportions depending on the tissue examined (Lands et al. 1967, Carlsson et al. 1972, Barnett et al. 1978, Minneman et al. 1979a). The concentrations of the two subtypes are regulated independently (Minneman et al. 1979b) and their molecular structure seems to be different (Venter et al. 1981). The β₁, -subtype is dominating in normal rat parotid glands (Carlsöö et al. 1981, Ludford & Talamo 1980). Supersensitivity occuring in salivary glands after interruption of the sympathetic nerve supply in adult animals is usually associated with an increased number of the β-adrenoceptors (Ludford & Talamo 1980, Stefano & Perec 1981). In the present work we have studied the effect of neonatal sympathetic denervation of the rat parotid gland on the binding of the β-adrenoceptor radioligand ³H-dihydroalprenolol (³H-DHA) and on the β-adrenoceptor-induced amylase secretion. The results suggest that the normal dominance of the β₁-adrenoceptors is lost after neonatal sympathetic denervation. Hence, an intact sympathetic nervous system may be of major importance during the development of the β-adrenoceptor population.     

HLA-SB in the south of France. Correlation between locally derived and reference typing reagents

The HLA-D region of the Major Histocompatibility Complex has been subdivided since 1978 (Mawas et al. 1978) into two subregions separable by recombination: a telomeric subregion (closer to HLA-B), coding for the classical HLA-DR or Dw specificities (Mawas et al. 1980) as well as for the more recent MT series (Park et al. 1980); and a centromeric subregion (closer to GLO), coding for a new series of alleles provisionally named SB (for secondary B cell antigens) (Shaw et al. 1980, 1981a). Reagents allowing the identification of six independent alleles have been characterized in two laboratories (Charmot et al. 1980 and Shaw et al. 1980, 1981b) using the technology of primed lymphocytes typing (Sheehy et al. 1975; Mawas et al. 1975). The existence of this new locus is supported by the following arguments: population studies by Shaw demonstrating five traits distinct from DR behaving as alleles (Shaw et al. 1981b), analysis of two informative SB/DR recombinant families (Mawas et al. 1978; Mawas et al. 1980; Shaw et al. 1981a), and, finally, studies of mutants showing independent loss of DR expression without loss of SB expression (Kavathas et al. 1981). The present report summarizes the HLA-SB typing of 109 unrelated individuals from the South of France and segregation studies in 14 unrelated families; a first attempt to correlate local "SB" reagents with the NIH reference standards is presented.     

Analysis of a DNA segment from rat liver mitochondria containing the genes for the cytochrome oxidase subunits I,II and III,ATPase subunit 6, and several tRNA genes

Summary The sequence of a 6.24 kb DNA segment of the mitochondrial genome from rat liver has been determined. It comprises several genes coding for mitochondrial protein subunits and five tRNA genes in the following order: cytochrome oxidase subunit I — tRNA _(UCN) ^Ser —tRNA^Asp — cytochrome oxidase subunit II — tRNALys —ATPase subunit — cytochrome oxidase subunit III —tRNA^Gly — potential open reading frame — tRNA^Arg —two potential open reading frames. The tRNA genes were detected by a computer search programme. The assignments for the protein coding sequences were made through comparison with known sequences, mainly from the yeast mitochondrial proteins (e.g. Bonitz et al. 1980). Our data are discussed with regard to the features of gene arrangement, codon usage, and tRNA structure in mammalian mitochondria (Anderson et al. 1981).Abbreviations COX I, COX II, COX III mitochondrial cytochrome oxidase subunits I, II, and III - ATPase mitochondrial ATPase subunit 6 - U.R.F. unidentified reading frame (Anderson et al. 1981). Other abbreviations follow IUB-IUPAC conventions.     

Chromatographic analysis of the aminoacyl-tRNAs which are required for translation of codons at and around the ribosomal frameshift sites of HIV, HTLV-1, and BLV

An examination of the frameshift signals or proposed signals within published sequences of retroviruses and other genetic elements from higher animals shows that each site utilizes a tRNA which normally contains Wybutoxine (Wye) base or Queuine (Q) base in the anticodon loop. We find experimentally that most of the Phe-tRNA present in HIV-1 infected cells lacks the highly modified Wye base in its anticodon loop and most of the Asn-tRNA in HTLV-1 and BLV infected cells lacks the highly modified Q base in its anticodon loop. Interestingly, Phe-tRNA translates a UUU codon within the ribosomal frameshift signal in HIV and Asn-tRNA translates a AAC codon within the proposed frameshift signals in HTLV-1 and BLV. Thus, the lack of a highly modified base in the anticodon loop of tRNAs in retroviral infected cells is correlated with the participation of these undermodified tRNAs in the corresponding frameshift event. This suggests that the "shifty" tRNAs proposed by Jacks et al. (Cell 55, 447-458, 1988) to carry out frameshifting may be hypomodified isoacceptors.     

About biased and non-biased transmission of chloroplast genes following artificial fusion of gametes in Chlamydomonas reinhardtii

Summary Artificial polyethyleneglycol induced fusions of gametes of opposite mating-types carrying chloroplast markers give rise to fusion products transmitting either both markers or the marker from the mt ⁺ or from the mt ^– parent exclusively. The frequencies of the three classes of products were approximately equal in our experiments (Matagne 1981). Similar experiments performed by Matsuda et al. (1983) gave different results, namely a preferential transmission of chloroplast gene from the mt ⁺ parent, very similar to that observed in vegetative zygotes obtained in sexual crosses. Results described here show that in experimental conditions used by Matsuda et al., sexual copulation does occur, leading to formation of zygotes which were misinterpreted as artificial fusion products and gave a biased transmission of chloroplast genes.     

DNA repair and recombination functions in <Emphasis Type="Italic">Arabidopsis</Emphasis> telomere maintenance

In this review, we discuss recent advances in the knowledge of plant telomere maintenance, focusing on the model plant Arabidopsis thaliana and, in particular, on the roles of proteins involved in DNA repair and recombination. The question of the interrelationships between DNA repair and recombination pathways and proteins with telomere function and maintenance is of increasing interest and has been the subject of a number of recent reviews (Cech 2004, d’Adda di Fagagna et al. 2004, Hande 2004, Harrington 2004, Maser & DePinho 2004). Understanding of telomere biology, DNA repair and recombination in plants has rapidly progressed over the last decade, substantially due to genetic approaches in Arabidopsis, and we feel that this is an appropriate time to review current knowledge in this field. A number of recent reviews have dealt more generally with the subject of plant telomere structure and evolution (Riha et al. 2001, McKnight et al. 2002, Riha & Shippen 2003b, McKnight & Shippen 2004, Fajkus et al. 2005) and we thus focus specifically on plant telomere biology in the context of DNA repair and recombination in Arabidopsis.     

Characterization of a mutation in Saccharomyces cerevisiae that produces mutant isoaccepting tRNAs for several of its tRNA species

Summary Genetic data presented in Bell et al. (1977) demonstrated that a mutation in Saccharomyces cerevisiae (designated mia) is responsible for the production of mutant isoaccepting tRNA molecules for some tRNA species. Besides extending this phenotype to other tRNAs, we have shown that mutant isoacceptors are produced at the expense of the normal levels of wild type tRNA and that the presence of mutant isoacceptors has no adverse effects on strains harbouring the mutation. The observation that mia strains have mutant isoacceptors as the predominant tRNA species under certain growth conditions suggests that mutant isoacceptors are biologically active molecules. Pulse-label and chase experiments indicate that mutant isoacceptors are slowly converted to wild type tRNA molecules in vivo, suggesting that they could be precursor molecules. This is consistent with the hypothesis that mia is defective in a modification process in the maturation of tRNA molecules. Analysis of a double mutant that produces mia isoacceptors which also lack N²-dimethylguanine shows that some of the modifications to tRNA molecules need not follow a specific sequence.     

Suppressor specificity in Aspergillus nidulans

Summary Seven allele specific gene unspecific suppressors mapping at four loci have been described previously (Roberts et al. 1979). Three new suppressors mapping in suaA are characterised, and the spectrum of suppression of all the suppressors with respect to seventeen suppressible mutations in eight different genes is described. Two distinct classes of suppressor are defined. The diversity of suppression of five suaA alleles, and the temperature sensitivity of some suaA suppressor mutant combinations but not others, suggests that suppressors at this locus are acting via ribosomal protein alteration. suaC109, a mutation that results in cold-sensitivity for growth shows a similar broad spectrum of suppression. Suppressors at the suaA and suaC loci suppress mutations that have the properties of chain termination mutations as well as missense mutations. suaB111, and suaD103 and suaD108 have a very restricted range of suppression. These suppressors may be mutations in tRNA genes.     

Mosquito mitochondrial transfer RNAs for valine,glycine and glutamate: RNA and gene sequences and vicinal genome organization 19851230

Summary We report the sequences of 3 transfer RNAs from mosquito (Aedes albopictus) mitochondria, those for valine (anticodon UAC), glutamic acid (anticodon UUC) and glycine (anticodon UCC), as well as sequences for the corresponding genes and for some neighboring mitochondrial genes. TRNA^va1 is notable for its high level of , tRNA^glu for its low level of G and C, and tRNA^gly is notable in that it appears as two species widely separated in gel electrophoresis, differing only in modification status. TRNA^glu is the first sequenced insect mitochondrial tRNA that would be expected to engage in U^* · R wobble (where U^* is a modified U in the first position of the anticodon, and R is G or A in the third position of codons), if the insect system followed the modified wobble rules proposed for mammalian and fungal mitochondria; and the sequence determined does fit the proposal. The gene for tRNA^va1 follows immediately that for 12S ribosomal RNA. The gene for tRNA^glu occurs in a cluster of 6 tRNA genes that is separated from the gene for tRNA^gly by a short reading frame. Features of the DNA sequences are discussed with reference to Drosophila, and mammalian, mitochondrial genome organization.