Global patterns of human DNA sequence variation in a 10-kb region on chromosome 1.

Human DNA variation is currently a subject of intense research because of its importance for studying human origins, evolution, and demographic history and for association studies of complex diseases. A approximately 10-kb region on chromosome 1, which contains only four small exons (each <155 bp), was sequenced for 61 humans (20 Africans, 20 Asians, and 21 Europeans) and for 1 chimpanzee, 1 gorilla, and 1 orangutan. We found 52 polymorphic sites among the 122 human sequences and 382 variant sites among the human, chimpanzee, gorilla, and orangutan sequences. For the introns sequenced (8,991 bp), the nucleotide diversity (pi) was 0.058% among all sequences, 0.076% among the African sequences, 0.047% among the Asian sequences, and 0.045% among the European sequences. A compilation of data revealed that autosomal regions have, on average, the highest pi value (0.091%), X-linked regions have a somewhat lower pi value (0.079%), and Y-linked regions have a very low pi value (0.008%). The lower polymorphism in the present region may be due to a lower mutation rate and/or selection in the gene containing these introns or in genes linked to this region. The present region and two other 10-kb noncoding regions all show a strong excess of low-frequency variants, indicating a relatively recent population expansion. This region has a low mutation rate, which was estimated to be 0.74 x 10 per nucleotide per year. An average estimate of approximately 12,600 for the long-term effective population size was obtained using various methods; the estimate was not far from the commonly used value of 10,000. Fu and Li's tests rejected the assumption of an equilibrium neutral Wright-Fisher population, largely owing to the high proportion of low-frequency variants. The age of the most recent common ancestor of the sequences in our sample was estimated to be more than 1 Myr. Allowing for some unrealistic assumptions in the model, this estimate would still suggest an age of more than 500,000 years, providing further evidence for a genetic history of humans much more ancient than the emergence of modern humans. The fact that many unique variants exist in Europe and Asia also suggests a fairly long genetic history outside of Africa and argues against a complete replacement of all indigenous populations in Europe and Asia by a small Africa stock. Moreover, the ancient genetic history of humans indicates no severe bottleneck during the evolution of humans in the last half million years; otherwise, much of the ancient genetic history would have been lost during a severe bottleneck. We suggest that both the "Out of Africa" and the multiregional models are too simple to explain the evolution of modern humans.

Members of the Arabidopsis 14-3-3 gene family trans-complement two types of defects in fission yeast.

14-3-3 proteins are highly conserved among eukaryotes and perform diverse biochemical activities. We isolated five types of Arabidopsis 14-3-3 cDNAs in a screen for clones that could block ectopic meiosis driven by the pat1 mutation in fission yeast. Overexpression of fission yeast rad24, which encodes a 14-3-3 protein, also suppressed pat1. All Arabidopsis clones isolated could rescue the deformed morphology and elevated UV sensitivity of the rad24 mutant. Thus, it appears that Arabidopsis 14-3-3 proteins can generally substitute for their fission yeast counterpart in function. Expression of an Arabidopsis 14-3-3 clone, GF14µ, was shown to be rather ubiquitous among plant organs.

Genetic definition and sequence analysis of Arabidopsis centromeres.

High-precision genetic mapping was used to define the regions that contain centromere functions on each natural chromosome in Arabidopsis thaliana. These regions exhibited dramatic recombinational repression and contained complex DNA surrounding large arrays of 180-base pair repeats. Unexpectedly, the DNA within the centromeres was not merely structural but also encoded several expressed genes. The regions flanking the centromeres were densely populated by repetitive elements yet experienced normal levels of recombination. The genetically defined centromeres were well conserved among Arabidopsis ecotypes but displayed limited sequence homology between different chromosomes, excluding repetitive DNA. This investigation provides a platform for dissecting the role of individual sequences in centromeres in higher eukaryotes.

Functional cloning of a cDNA encoding Mei2-like protein from Arabidopsis thaliana using a fission yeast pheromone receptor deficient mutant.

To isolate Arabidopsis cDNAs that encode signal transducers and components involved in the regulation of meiosis, a trans-complementation analysis was performed using a Schizosaccharomyces pombe meiosis-defective mutant in which the genes for pheromone receptors were disabled. One cDNA obtained in this screening encodes a polypeptide, named AML1, that shows significant similarity to S. pombe Mei2 protein and has three putative RNA-recognition motifs like as Mei2. Mei2 is involved in the regulation of meiosis in fission yeast. Northern blot analysis showed that the AML1 gene is expressed in each organ. The possible functions of AML1 are discussed.

Cloning of cDNAs from Arabidopsis thaliana that encode putative protein phosphatase 2C and a human Dr1-like protein by transformation of a fission yeast mutant.

We characterized three Arabidopsis thaliana cDNA clones that could rescue the sterile phenotype of the Schizosaccharomyces pombe pde1 mutant, which is defective in cAMP phosphodiesterase. The first clone had a coding capacity of 399 amino acids that is 35% identical with rat protein phosphatase 2C (PP2C). The second had a coding capacity of 159 amino acids that is 41% identical with human Dr1. Dr1 has been shown to interact with TATA-binding protein (TBP) and block its ability to activate transcription. The third encoded Arabidopsis TBP itself. Saccharomyces cerevisiae TBP also could suppress the sterile phenotype if expressed in S.pombe pde1 cells, but overexpression of S.pombe TBP could do so very poorly. These observations suggest preliminarily that PP2C may counteract cAMP-dependent protein kinase in fission yeast cells, and that the heterologous TBPs and Dr1 may interfere with the general transcription factors of S.pombe so that the gene expression in the host cell becomes affirmative of sexual development. Furthermore, the identification of a Dr1-like protein in A.thaliana strongly argues for the ubiquity of this protein among eukaryotic genera and for a conserved mechanism to regulate transcription initiation that involves Dr1.