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Molecular cloning and nucleotide sequence of a new P450 gene, CYP319A1, from the cattle tick, Boophilus microplus.

We have isolated and sequenced a novel P450 gene (CYP319A1) from the cattle tick, Boophilus microplus. The CYP319A1 cDNA encodes a protein of 531 amino acids with an estimated molecular weight of 60.9k. It contains all highly conserved motifs characteristic of P450 enzymes. Comparison of deduced amino acid sequence with other CYP members shows that the CYP319A1 is more closely related to CYP4 family, but its overall identity to the CYP4 family is less than 40%. Therefore, it was assigned to a new P450 family by the P450 nomenclature committee. A pseudogene which shares high homology with the CYP319A1 was identified. Analysis of genomic sequence of the pseudogene indicated that the pseudogene contains two additional DNA inserts in the coding region, which disrupt the open reading frame. RT-PCR analysis showed that CYP319A1 is expressed in both susceptible and acaricide-resistant ticks.

Amino Acid Sequence↗

Alternate paths of evolution for the photosynthetic gene rbcL in four nonphotosynthetic species of Orobanche.

We have determined the nucleotide sequence for the Rubisco large subunit from four holoparasitic species of Orobanche. Intact open reading frames are present in two species (O. corymbosa and O. fasciculata), whereas the remaining species (O. cernua and O. ramosa) have rbcL pseudogenes. Sequences for rbcL 5'-UTRs from species of Orobanche have few changes in the promoter and ribosome binding sites compared to photosynthetic higher plants. Comparison of rbcL 3'-UTR sequences for Nicotiana, Ipomoea, Cuscuta, and Orobanche reveal that nucleotide sequences from parasitic plants have regions capable of forming stem-loop structures, but 56-69 nt are deleted upstream of the stem-loop in the parasitic plants compared to their photosynthetic relatives. Although rbcL pseudogenes of O. cernua and O. ramosa have many large and small deletions, few indels are shared in common, implying that their common ancestor probably had an intact rbcL reading frame. Intact rbcL reading frames in O. corymbosa and O. fasciculata retain a bias of synonymous over nonsynonymous substitutions and deduced protein sequences are consistent with potentially functional Rubisco large subunit proteins. A conservative model of random substitution processes in pseudogene sequences estimates that the probability is low (P < 0.028) that these sequences would retain an open reading frame by chance. Species of Orobanche have either had recent photosynthetic ancestors, implying multiple independent losses of photosynthesis in this genus, or the rbcL gene may serve an unknown function in some nonphotosynthetic plants.

Amino Acid Sequence↗

Insertions and duplications of mtDNA in the nuclear genomes of Old World monkeys and hominoids.

Using oligonucleotide primers designed to match conserved regions of mammalian mitochondrial DNA (mtDNA), we have amplified and sequenced two divergent cytochrome b nuclear pseudogenes from orangutan cellular DNA. Evolutionary analysis suggests that a nuclear transfer occurred about 30 million years ago on the lineage leading to the catarrhines (Old World monkeys and hominoids), and involved a long (at least 3 kilobases), probably damaged, piece of mtDNA. After this transfer, the pseudogene duplicated, giving rise to the two copies that are probably present in all hominoids, including humans. More recent transfers involving the entire cytochrome b gene have also occurred in the Old World monkeys. Such nuclear copies of mtDNA can confound phylogenetic and population genetic studies, and be an insidious source of DNA contamination of 'ancient' and forensic DNA. Indeed, contamination with these anciently transferred human pseudogenes is almost certainly the source of the cytochrome b sequences recently reported from 'dinosaur bone DNA'.

Animals↗

High mutation rate and predominance of insertions in the Caenorhabditis elegans nuclear genome.

Mutations have pivotal functions in the onset of genetic diseases and are the fundamental substrate for evolution. However, present estimates of the spontaneous mutation rate and spectrum are derived from indirect and biased measurements. For instance, mutation rate estimates for Caenorhabditis elegans are extrapolated from observations on a few genetic loci with visible phenotypes and vary over an order of magnitude. Alternative approaches in mammals, relying on phylogenetic comparisons of pseudogene loci and fourfold degenerate codon positions, suffer from uncertainties in the actual number of generations separating the compared species and the inability to exclude biases associated with natural selection. Here we provide a direct and unbiased estimate of the nuclear mutation rate and its molecular spectrum with a set of C. elegans mutation-accumulation lines that reveal a mutation rate about tenfold higher than previous indirect estimates and an excess of insertions over deletions. Because deletions dominate patterns of C. elegans pseudogene variation, our observations indicate that natural selection might be significant in promoting small genome size, and challenge the prevalent assumption that pseudogene divergence accurately reflects the spontaneous mutation spectrum.

Animals↗

Regional evolution of venom-gland phospholipase A2 isoenzymes of Trimeresurus flavoviridis snakes in the southwestern islands of Japan.

Conventional chromatographic analysis showed that phospholipase A(2) (PLA(2)) isoenzymes of the venom of Trimeresurus flavoviridis (Habu snake) of Okinawa island are profoundly different in composition from those of T. flavoviridis of Amami-Oshima and Tokunoshima islands. The most striking feature was that myotoxic [Lys(49)]PLA(2) isoenzymes, called BPI and BPII, which are expressed abundantly in the venoms of Amami-Oshima and Tokunoshima T. flavoviridis, are missing from the venom of Okinawa T. flavoviridis. Northern blot analysis of Okinawa T. flavoviridis venom-gland mRNA species showed the absence of BPI and BPII mRNA species. Analysis by single-stranded conformational polymorphism-PCR of venom-gland mRNA species of T. flavoviridis from three islands, with reference to five DNA species each encoding different PLA(2) isoenzymes from Tokunoshima T. flavoviridis venom gland, also suggested that BPI and BPII mRNA species are not expressed in Okinawa T. flavoviridis venom gland. In contrast, genomic Southern blot analysis with a variety of probes showed that only the bands corresponding to the upstream and downstream regions of the genes for BPI and/or BPII can be detected in Okinawa T. flavoviridis. These results suggested that the genes for BPI and BPII in Okinawa T. flavoviridis genome had been inactivated to form pseudogenes. Differently from Amami-Oshima and Tokunoshima T. flavovirdis genomic DNAs, PCR amplification of the segments of BPI and BPII genes between the 5' moiety of second exon and the middle portion of second intron failed for Okinawa T. flavoviridis genomic DNAs. In sequence analysis of the two segments involving polymorphism between BPI and BPII genes, which are located in first exon and third exon, respectively, only one base was detected at the polymorphic positions for pseudogene in Okinawa T. flavoviridis genome. Based on these facts, it became evident for pseudogene that the upstream region of BPI gene down to the 5' moiety of second exon and the downstream region of BPII gene starting from the middle portion of second intron are in a linked form with a possible insertion. Such observations suggest that venom-gland genes for PLA(2) isoenzymes in T. flavoviridis snakes isolated for one to two million years have evolved independently. Their evolution is regional and seems, from several lines of consideration and observation, to be adaptive to the environment.

Animals↗

Conservation of a gene conversion mechanism in two distantly related paralogues of Anaplasma marginale.

Anaplasmataceae, the causative agents of anaplasmosis and ehrlichiosis, persist in the bloodstream of their mammalian hosts, allowing acquisition and transmission by tick vectors. Anaplasma marginale establishes persistent infection characterized by sequential cycles of rickettsaemia in which new antigenic variants emerge. The two most immunodominant outer membrane proteins, MSP2 and MSP3, are paralogues, each encoded by a distinct family of related genes. This study demonstrates that, although the two gene families have diverged substantially, each has maintained a similar mechanism to generate structurally and antigenically polymorphic surface antigens. Like MSP2, MSP3 is expressed from a single locus in which variation of the expressed msp3 gene is generated by recombination using msp3 pseudogenes. Each of the msp3 pseudogenes encodes a unique central variable region (CVR) flanked by conserved 5' and 3' regions. Changes in the CVR of the expressed msp3, concomitant with invariance of the pseudogenes, indicate that expression site variation is generated using gene conversion. A. marginale thus maintains two large, separate systems within its small genome to generate antigenic variation of its surface proteins, while analogous structural elements indicate a common mechanism.

Acute Disease↗

Identification, structure and differential expression of novel pleurocidins clustered on the genome of the winter flounder, Pseudopleuronectes americanus (Walbaum).

Antimicrobial peptides form one of the first lines of defense against invading pathogens by killing the microorganisms and/or mobilizing the host innate immune system. Although over 800 antimicrobial peptides have been isolated from many different species, especially insects, few have been reported from marine fish. Sequence analysis of two genomic clones (15.6 and 12.5 kb) from the winter flounder, Pseudopleuronectes americanus (Walbaum) resulted in the identification of multiple clustered genes for novel pleurocidin-like antimicrobial peptides. Four genes and three pseudogenes (Psi) are encoded in these clusters, all of which have similar intron/exon boundaries but specify putative antimicrobial peptides differing in sequence. Pseudogenes are easily detectable but have incorrect initiator codons (ACG) and often contain a frameshift(s). Potential promoters and binding sites for transcription factors implicated in regulation of expression of immune-related genes have been identified in upstream regions by comparative genomics. Using reverse transcription-PCR assays, we have shown for the first time that each gene is expressed in a tissue-specific and developmental stage-specific manner. In addition, synthetic peptides based on the sequences of both genes and pseudogenes have been produced and tested for antimicrobial activity. These data can be used as a basis for prediction of antimicrobial peptide candidates for both human and nonhuman therapeutants from genomic sequences and will aid in understanding the evolution and transcriptional regulation of expression of these peptides.

Amino Acid Sequence↗

Human specific loss of olfactory receptor genes.

Olfactory receptor (OR) genes constitute the basis for the sense of smell and are encoded by the largest mammalian gene superfamily of >1,000 genes. In humans, >60% of these are pseudogenes. In contrast, the mouse OR repertoire, although of roughly equal size, contains only approximately 20% pseudogenes. We asked whether the high fraction of nonfunctional OR genes is specific to humans or is a common feature of all primates. To this end, we have compared the sequences of 50 human OR coding regions, regardless of their functional annotations, to those of their putative orthologs in chimpanzees, gorillas, orangutans, and rhesus macaques. We found that humans have accumulated mutations that disrupt OR coding regions roughly 4-fold faster than any other species sampled. As a consequence, the fraction of OR pseudogenes in humans is almost twice as high as in the non-human primates, suggesting a human-specific process of OR gene disruption, likely due to a reduced chemosensory dependence relative to apes.

Animals↗

Functionally important glycosyltransferase gain and loss during catarrhine primate emergence.

A glycosyltransferase, alpha1,3galactosyltransferase, catalyzes the terminal step in biosynthesis of Galalpha1,3Galbeta1-4GlcNAc-R (alphaGal), an oligosaccharide cell surface epitope. This epitope or antigenically similar epitopes are widely distributed among the different forms of life. Although abundant in most mammals, alphaGal is not normally found in catarrhine primates (Old World monkeys and apes, including humans), all of which produce anti-alphaGal antibodies from infancy onward. Natural selection favoring enhanced resistance to alphaGal-positive pathogens has been the primary reason offered to account for the loss of alphaGal in catarrhines. Here, we question the primacy of this immune defense hypothesis with results that elucidate the evolutionary history of GGTA1 gene and pseudogene loci. One such locus, GGTA1P, a processed (intronless) pseudogene (PPG), is present in platyrrhines, i.e., New World monkeys, and catarrhines but not in prosimians. PPG arose in an early ancestor of anthropoids (catarrhines and platyrrhines), and GGTA1 itself became an unprocessed pseudogene in the late catarrhine stem lineage. Strong purifying selection, denoted by low nonsynonymous substitutions per nonsynonymous site/synonymous substitutions per synonymous site values, preserved GGTA1 in noncatarrhine mammals, indicating that the functional gene product is subjected to considerable physiological constraint. Thus, we propose that a pattern of alternative and/or more beneficial glycosyltransferase activity had to first evolve in the stem catarrhines before GGTA1 inactivation could occur. Enhanced defense against alphaGal-positive pathogens could then have accelerated the replacement of alphaGal-positive catarrhines by alphaGal-negative catarrhines. However, we emphasize that positively selected regulatory changes in sugar chain metabolism might well have contributed in a major way to catarrhine origins.

Animals↗

Multiple retropseudogenes from pluripotent cell-specific gene expression indicates a potential signature for novel gene identification.

Oct4, Nanog, and Stella are transcription factors specifically expressed in embryonic stem (ES) cells and germ lineage cells that impart critical functions in the maintenance of pluripotency. Here, we report the excessive frequency and apparent selectivity of retrotransposition of ES cell-specific genes. Six highly homologous pseudogenes for Oct4, 10 for Nanog, and 16 for Stella were identified by nucleotide BLAST (basic local alignment sequence tool) searches against the respective gene mRNA transcripts. Of 15 non-ES cell-specific transcription factor genes, only one had a single pseudogene hit in our screen, emphasizing the apparent selectivity. We present a hypothesis whereby retrotransposition of ES or germ cell-specific genes may reflect an innate predisposition. This is based on the increased probability of germ-line transmission when retrotransposition occurs at a very early stage of development within cells known to contribute to the germ cell lineage. The parental genes for Nanog, Stella, and another embryonic gene, GDF3 are all located on chromosome 12p13 of the human genome, and on chromosome 6 in mouse. Here, we identified an Oct4 pseudogene at the same respective loci in both human and mouse genomes, suggesting functional relevance and indicative of epigenetic regulation. We tested whether the apparent susceptibility for ES cell-specific gene retrotransposition may be extrapolated to a more unified phenomenon, such that a bioinformatic approach may represent a potentially novel strategy for identification of genes with embryonic cell-specific functionality. A preliminary investigation indeed revealed a single gene, previously demonstrated to be responsible for multiple retropseudogenes via germ cell-specific expression in Xenopus.

Chromosomal Proteins, Non-Histone↗

Molecular cloning of mouse somatic and testis-specific H2B histone genes containing a methylated CpG island.

We have isolated a mouse testis-specific H2B histone gene based on the unusual methylation of the CpG island of rat testis-specific H2B gene in somatic tissues. After digestion of genomic DNA with the methylation-sensitive restriction enzyme Hha I, we found that, among 10-20 copies of mouse H2B histone genes, at least three copies are methylated in somatic tissues, but not in testis. Cloning and sequence analysis of two methylated H2B genes revealed that one gene, MTH2B, is strikingly similar to the testis-specific histone H2B (TH2B) gene of rat and the other, psH2B, is a pseudogene of the somatic-type H2B gene. Northern blot analysis revealed that the expression of the MTH2B gene is testis-specific. During spermatogenesis, the MTH2B gene is expressed predominantly in pachytene spermatocytes, as observed in the expression of rat TH2B gene. Interestingly, the MTH2B gene is largely unmethylated in embryonic stem cells, but methylated in F9 embryonal carcinoma cells. The psH2B pseudogene is methylated in somatic tissues and F9 cells, but only partially methylated in embryonic stem cells. Methylation of the psH2B pseudogene seems to be attributed to its location within the context of repetitive sequences including the B1 element. The unmethylation of both H2B histone genes in the testis explains how CpG islands of those histone genes can be maintained during evolution despite heavy methylation in somatic tissues.

Amino Acid Sequence↗

Structure and evolution of the Adh genes of Drosophila mojavensis.

The nucleotide sequence of the Adh region of Drosophila mojavensis has been completed and the region found to contain a pseudogene, Adh-2 and Adh-1 arranged in that order. Comparison of the sequence divergence of these genes to one another and to the Adh region of Drosophila mulleri and other species has allowed the development of a model for the evolution of the duplication of the Adh genes. There have been two major events. An initial duplication of an Adh gene whose dual promoter structure was similar to Drosophila melanogaster, resulted in a species with two Adh genes, one of which may have had only a proximal promoter. A second duplication of this gene generated an Adh region containing three genes. It is proposed that one of these is the ancestral gene having dual promoters, while the other two possess only proximal promoters. Subsequent events have resulted in both a change in the regulation of Adh-2 such that it is expressed as if it had a "distal" type promoter and the mutational inactivation of the most upstream gene resulting in the creation of a pseudogene. The sequence of the D. mojavensis Adh region has also revealed the presence of an element which is composed of juxtaposed inverted imperfectly repeated elements. There is a surprising and not fully explainable strong similarity of the nucleotide sequence of the 5' flanking region of the pseudogene in D. mojavensis and D. mulleri.

Alcohol Dehydrogenase↗

How often do duplicated genes evolve new functions?

A recently duplicated gene can either fix a null allele (becoming a pseudogene) or fix an (advantageous) allele giving a slightly different function, starting it on the road to evolving a new function. Here we examine the relative probabilities of these two events under a simple model. Null alleles are assumed to be neutral; linkage effects are ignored, as are unequal crossing over and gene conversion. These assumptions likely make our results underestimates for the probability that an advantageous allele is fixed first. When new advantageous mutations are additive with selection coefficient s and the ratio of advantageous to null mutations is rho, the probability an advantageous allele is fixed first is ([1 - e-s]/[rho S] + 1)-1, where S = 4Nes with Ne the effective population size. The probability that a duplicate locus becomes a pseudogene, as opposed to evolving a new gene function, is high unless rhoS >> 1. However, even if advantageous mutations are very rare relative to null mutations, for sufficiently large populations rhoS >> 1 and new gene function, rather than pseudogene formation, is the expected fate of most duplicated genes.

Biological Evolution↗

Estimate of the mutation rate per nucleotide in humans.

Many previous estimates of the mutation rate in humans have relied on screens of visible mutants. We investigated the rate and pattern of mutations at the nucleotide level by comparing pseudogenes in humans and chimpanzees to (i) provide an estimate of the average mutation rate per nucleotide, (ii) assess heterogeneity of mutation rate at different sites and for different types of mutations, (iii) test the hypothesis that the X chromosome has a lower mutation rate than autosomes, and (iv) estimate the deleterious mutation rate. Eighteen processed pseudogenes were sequenced, including 12 on autosomes and 6 on the X chromosome. The average mutation rate was estimated to be approximately 2.5 x 10(-8) mutations per nucleotide site or 175 mutations per diploid genome per generation. Rates of mutation for both transitions and transversions at CpG dinucleotides are one order of magnitude higher than mutation rates at other sites. Single nucleotide substitutions are 10 times more frequent than length mutations. Comparison of rates of evolution for X-linked and autosomal pseudogenes suggests that the male mutation rate is 4 times the female mutation rate, but provides no evidence for a reduction in mutation rate that is specific to the X chromosome. Using conservative calculations of the proportion of the genome subject to purifying selection, we estimate that the genomic deleterious mutation rate (U) is at least 3. This high rate is difficult to reconcile with multiplicative fitness effects of individual mutations and suggests that synergistic epistasis among harmful mutations may be common.

Animals↗

A complete alpha1,3-galactosyltransferase gene is present in the human genome and partially transcribed.

The synthesis of Galalpha1-3Gal-terminated oligosaccharides (alpha-Gal) epitopes has been interrupted during the course of evolution, starting with Old World primates. Partial sequences similar to the alpha1,3-galactosyltransferase (alpha1,3GalT) gene, which governs the synthesis of alpha-Gal epitopes, have been detected in the human genome and were found to correspond to pseudogenes. We completed the sequence of the human alpha1,3GalT pseudogene present on chromosome 9 and found it to be organized like the murine alpha1,3GalT gene. In human cell lines and several normal and tumor tissues we detected truncated transcripts corresponding to this pseudogene. Considering these mRNAs, translation of an open reading frame containing the first four translated exons but missing the two catalytic exons could predict a truncated alpha1,3GalT polypeptide that should be enzymatically inactive. We show that transcription of human alpha1,3GalT is prematurely terminated at the level of a strong transcriptional stop signal in the middle of intron VII. We were able to reproduce this effect in vitro by subcloning the implicated DNA region upstream from a reporter cDNA. The premature transcriptional arrest of human alpha1,3-GalT gene leads to an ectopic splicing event and to the connection of a short intronic sequence downstream from translated exons. Finally, we show that these truncated transcripts are overexpressed in cell lines with modifications of O-glycans.

Base Sequence↗

The -104G nucleotide of the human CYP21 gene is important for CYP21 transcription activity and protein interaction.

CYP21 gene encodes the steroid 21-hydroxylase (P450c21) that is involved in steroidogenesis in the adrenal cortex. Mutations occurring on CYP21 which convert it to the neighboring pseudogene, CYP21P, are found in patients with congenital adrenal hyperplasia (CAH), an autosomal recessive disease. We previously reported that the CYP21P pseudogene had lower transcription activity when compared with the active CYP21 gene. The sequences determining the basal transcription activity of the human CYP21 gene are located within the 166 bp region upstream from the transcriptional start site. Within this region, only 4 nucleotides are different between the active CYP21 and the CYP21P pseudogene; they are located at the -117, -104, -101 and -94 positions from the start site of the gene. Here, we report that the CYP21 gene-specific G nucleotide sequence at the -104 position is required for its basal transcription activity driven by the native TATA box of the human CYP21 gene. When this single nucleotide is changed to the CYP21P sequence, the basal transcription activity decreases by approximately 80% in transient transfection assay. In addition, our data from gel retardation assay show that this sequence is also critical for interaction with nuclear proteins from adrenal cells. These results therefore suggest that the single G sequence of the human CYP21 gene is crucial for the expression of its basal transcription activity, and this may be influenced by the interaction with specific nuclear proteins from the adrenal gland.

Adrenal Cortex↗

The genome sequence of Salmonella enterica serovar Choleraesuis, a highly invasive and resistant zoonotic pathogen.

Salmonella enterica serovar Choleraesuis (S. Choleraesuis), a highly invasive serovar among non-typhoidal Salmonella, usually causes sepsis or extra-intestinal focal infections in humans. S. Choleraesuis infections have now become particularly difficult to treat because of the emergence of resistance to multiple antimicrobial agents. The 4.7 Mb genome sequence of a multidrug-resistant S. Choleraesuis strain SC-B67 was determined. Genome wide comparison of three sequenced Salmonella genomes revealed that more deletion events occurred in S. Choleraesuis SC-B67 and S.Typhi CT18 relative to S. Typhimurium LT2. S. Choleraesuis has 151 pseudogenes, which, among the three Salmonella genomes, include the highest percentage of pseudogenes arising from the genes involved in bacterial chemotaxis signal-transduction pathways. Mutations in these genes may increase smooth swimming of the bacteria, potentially allowing more effective interactions with and invasion of host cells to occur. A key regulatory gene of TetR/AcrR family, acrR, was inactivated through the introduction of an internal stop codon resulting in overexpression of AcrAB that appears to be associated with ciprofloxacin resistance. While lateral gene transfer providing basic functions to allow niche expansion in the host and environment is maintained during the evolution of different serovars of Salmonella, genes providing little overall selective benefit may be lost rapidly. Our findings suggest that the formation of pseudogenes may provide a simple evolutionary pathway that complements gene acquisition to enhance virulence and antimicrobial resistance in S. Choleraesuis.

Bacterial Proteins↗

Structural and evolutionary studies on sterol 14-demethylase P450 (CYP51), the most conserved P450 monooxygenase: II. Evolutionary analysis of protein and gene structures.

Phylogenetic analyses based on protein sequence data indicated that sterol 14-demethylase P450 (CYP51) and bacterial CYP51-like protein were joined into a distinctive evolutionary cluster, CYP51 cluster, within the CYP protein superfamily. The most probable branch topology of the CYP51 phylogenetic tree was (bacteria, (plants, (fungi, mammals))), which is comparable to the phylogeny of major kingdoms of living matter, suggesting that CYP51 has been conserved from the era of prokaryotic evolution. This may be strong evidence supporting the prokaryotic origin of P450. Structure of flanking regions and the number and insertion sites of introns are quite different between mammalian and fungal CYP51s. This fact indicates that different mechanisms are operative in evolution of protein sequences and gene structures. CYP51 is the first example violating the well-documented rule that the basic structure of a gene, including intron insertion sites, is well conserved in each P450 family. One CYP51 processed a pseudogene was found in rat genome. Nonsynonymous nucleotide divergence observed between the pseudogene and CYP51 cDNA was less than one-fifth of the synonymous divergence. This unusually low rate of nonsynonymous nucleotide changes in the pseudogene suggests that it may be derived from another CYP51, which might have been active for a significant duration in the past.

Amino Acid Sequence↗