In backcrosses between D. koepferae and D. buzzatii, the disruption of a system of species-specific complementary factors brings about hybrid male inviability. This system consists of a lethal factor, hmi-1, linked to the X chromosome of D. koepferae, and several conspecific autosomal suppressors. However, hmi-1 hybrid males can also be rescued by factors present in some strains of D. buzzatii. The present work aims to estimate the number of hmi-1 suppressors in one of these strains by means of Markov chains. The obtained results allow discarding models with one or more chromosomes having independent suppressor effect. On the other hand, models having n chromosomes that interact in groups of r, being 1<r≤n, to produce rescue effect, provide good approximations to the observed results. The best fit to the data is obtained with four or five chromosomes with suppressor effect, interacting epistatically in groups of three to rescue the viability of hmi-1 males.
A repeating unit of the histone gene cluster from Drosophila simulans containing the H1, H2A, H2B and H4 genes (the H3 gene region has already been analyzed) was cloned and analyzed. A nucleotide sequence of about 4.6 kbp was determined to study the nucleotide divergence and molecular evolution of the histone gene cluster. Comparison of the structure and nucleotide sequence with those of Drosophila melanogaster showed that the four histone genes were located at identical positions and in the same directions. The proportion of different nucleotide sites was 6.3% in total. The amino acid sequence of H1 was divergent, with a 5.1% difference. However, no amino acid change has been observed for the other three histone proteins. Analysis of the GC contents and the base substitution patterns in the two lineages, D. melanogaster and D. simulans, with a common ancestor showed the following. 1) A strong negative correlation was found between the GC content and the nucleotide divergence in the whole repeating unit. 2) The mode of molecular evolution previously found for the H3 gene was also observed for the whole repeating unit of histone genes; the nucleotide substitutions were stationary in the 3’ and spacer regions, and there was a directional change of the codon usage to the AT-rich codons. 3) No distinct difference in the mode or pattern of molecular evolution was detected for the histone gene repeating unit in the D. melanogaster and D. simulans lineages. These results suggest that selectional pressure for the coding regions of histones, which eliminate A and T, is less effective in the D. melanogaster and D. simnulans lineages than in the other GC-rich species.
In addition to a unique tumor-inducing (Ti) plasmid, the plant pathogenic bacterium Agrobacterium tumefaciens has an unconventional chromosomal organization. Our previous studies on A. tumefaciens MAFF301001 revealed that it possesses a 2 Mb linear and a 2.8 Mb circular chromosome plus a 206.479 kbp Ti plasmid (pTi-SAKURA). In this study, a linkage map for the left half of its linear chromosome covering a 900 kbp region was constructed and the number of potential genes existing in the region was estimated. The linkage map consists of 31 BAC and 8 lambda phage recombinants without any gaps. It confirmed the size and all the structural landmarks indicated in the corresponding region of our previously constructed physical map for the linear chromosome. Sequencing analysis of the end-regions of each linking clone led to the identification of 6 genes and another 27 potential genes or ORFs, including genes and/or gene clusters responsible for homologous recombination (ruvB ), trehalose/maltose sugar transport (thuR, thuG) and alanine catabolism (dadR). Two virulence-related gene homologues (attK and celB), previously reported in the circular chromosome of a different strain of A. tumefaciens were found in this region. These findings will provide a ready-to-use linkage map for further functional analysis of the linear chromosome.
We isolated 11 rice genes homologous to the genes encoding auxin response factors (ARFs) in Arabidopsis. All of the genes encoded a well-conserved amino acid sequence in the N-terminal region, which is considered to be a DNA-binding domain (DBD). Phylogenetic analysis based on comparison of the DBDs indicated that rice has one or two closely related orthologs corresponding to a given respective ARF gene in Arabidopsis. We also analyzed the amino acid sequences of another conserved domain in the C-terminal conserved domain (CTD), which was shared by almost all the rice ARFs, with the exception of OsETTIN1 and OsETTIN2. These results agreed well with the evolutionary relationship deduced from the DBD comparison. In contrast to many ARFs, OsETTIN1 and OsETTIN2 do not contain the conserved C-terminal domain, but do share another consensus motif that is also found in Arabidopsis ETTIN. All of the above observations indicate that rice has functionally diversified ARF genes whose structures and functions correspond to those of various Arabidopsis ARFs, with one or two rice ARFs corresponding to a given Arabidopsis ARF. Thus, auxin signal transduction mechanisms may be well conserved between monocot and dicot plants.
KNOX homeodomain (HD) proteins encoded by KNOTTED1-like homeobox genes (KNOX genes) are considered to work as important regulators for plant developmental and morphogenetic events. We found that OSH3, one of the KNOX genes isolated from a cultivar of Oryza sativa (Nipponbare), encodes a novel HD, which has two amino acid substitutions at invariant positions. Sequence analysis of OSH3 from various domesticated and wild species of rice has revealed that these substitutions are distributed only in Japonica and Javanica type of O. sativa, two groups of domesticated rice in Asia. Surprisingly, nucleotide sequences in the first intron are almost conserved in the rice strains that have the substitutions at the invariant amino acids. Overexpression studies revealed that these invariant amino acids are critical for the function of OSH3 in vivo. The facts that these substitutions occurred specifically at the functionally important amino acids and the sequences are conserved in intron where neutral mutations accumulate suggest the substitutions at the invariant positions of OSH3 have been fixed by artificial selections during domestication. Based on these observations, we hypothesize that OSH3 is responsible for one of the traits that are selectively introduced during the domestication of most of Japonica and a part of Javanica type of rice.
Las21/Gpi7 contains a heavy-metal-associated motif at its N-terminus. When this motif was disrupted by amino acid substitution, the cells acquired weak copper-resistance. We found that the previously isolated las21 mutants were strongly resistant to copper. Metallothionein is necessary for the expression of the copper-resistance of the las21 mutants. However, hyper-production of metallothionein is unlikely to be the cause of copper-resistance of the las21 mutants. Copper-sensitive mutants (collectively called Cus mutants) were isolated from the las21Δ and characterized. One of the Cus genes was found to be PBS2, which encodes Hog1 MAP kinase kinase, indicating that the Hog1 MAP kinase pathway is needed for the expression of copper-resistance of the las21 mutants. As expected, the las21Δ hog1Δ strain was no longer copper-resistant. We found that Hog1 was constitutively activated in las21Δ cells and in ssk1Δ las21Δ cells but not in sho1Δ las21Δ cells. Inactivation of either FSR2/MCD4 or MPC1/GPI13, both of which are involved in GPI anchor synthesis, like LAS21, caused a similar level of constitutive activation of Hog1 kinase and copper-resistance as found in the las21Δ strain. The constitutive activation was canceled by introducing the ssk1 mutation, but not the sho1 mutation, in each GPI anchor mutant tested, suggesting that the defect in GPI anchor synthesis specifically affects the Sln1 branch of the MAP kinase pathway. Since the wild-type cells grown in YPD containing 0.5 M NaCl do not show copper-resistance, mere activation of Hog1 is not sufficient for expression of copper-resistance. We propose that a defect in GPI anchor synthesis has multiple consequences, including activation of the Hog1 MAP kinase cascade and conferring copper-reslstance.