Characterization of the gene conversions between the multigene family members of the yeast genome

Stanley Sawyer's gene conversion detection method, implemented in his GENECONV computer program, was used to detect and characterize the gene conversions between the multigene family members of the yeast genome. This method gave different gene conversion frequencies and size distribution for gene families with two members and multigene families with more than two members. The 69 gene conversions detected in multigene families with more than two members occur at a frequency of 7.8% gene conversion/pair of genes compared and have an average size of 173+/-220 nucleotides. Larger gene conversions are found only between more similar genes, the genes involved in gene conversions are distributed almost randomly among the 16 yeast chromosomes, and the frequency of gene conversions increases as the distance between repeated genes decreases. In contrast to previous studies, no relationship was observed between the level of expression of a gene and its involvement in gene conversions. These analyses also suggest that gene conversions might occur by different mechanisms in closely linked genes and unlinked genes. The excess of converted regions at the 3? end of unlinked genes suggests that recombination with incomplete cDNA molecules is the main mechanism responsible for gene conversions between such genes.     

Six family genes--structure and function as transcription factors and their roles in development

The members of the Six gene family were identified as homologues of Drosophila sine oculis which is essential for compound-eye formation. The Six proteins are characterized by the Six domain and the Six-type homeodomain, both of which are essential for specific DNA binding and for cooperative interactions with Eya proteins. Mammals possess six Six genes which can be subdivided into three subclasses, and mutations of Six genes have been identified in human genetic disorders. Characterization of Six genes from various animal phyla revealed the antiquity of this gene family and roles of its members in several different developmental contexts. Some members retain conserved roles as components of the Pax-Six-Eya-Dach regulatory network, which may have been established in the common ancestor of all bilaterians as a toolbox controlling cell proliferation and cell movement during embryogenesis. Gene duplications and cis-regulatory changes may have provided a basis for diverse functions of Six genes in different animal lineages.     

Divergence and differential expression of soybean actin genes

DNA sequence analysis as well as genomic blotting experiments using cloned soybean actin DNA sequences as probes show that large sequence heterogeneity exists among members of the soybean actin multigene family. This heterogeneity suggested that the members of this family might be diverged in function and/or regulation. Five of the six soybean actin gene family members examined are shown to be significantly more diverged from one another than members of other known actin gene families. This high level of divergence was utilized in the preparation of actin gene-specific probes in the analysis of the complexity and expression of these members of the soybean actin gene family. Hybridization studies indicate that the six soybean actin genes fall into three classes with a pair of genes in each class. These six genes account for all but two actin gene fragments detected in the soybean genome. We have compared the relative steady state mRNA levels of these classes of soybean actin genes in three organs of soybean. We find that actin genes SAc6 and SAc7 are most highly expressed accounting for 80% of all actin mRNA with respect to the six soybean actin genes examined. Actin genes SAc3 and SAc1 are expressed at intermediate and low levels respectively; and SAc2 and SAc4 are expressed at barely detectable levels. Four of the six soybean actin genes appear to be expressed at the same level in root, shoot and hypocotyl. SAc3 and SAc7 genes appear to be more highly expressed in shoot and 2,4-dichlorophenoxyacetic acid-induced hypocotyl than in root and hypocotyl.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heavy chain variable region gene families evolved early in phylogeny. Ig complexity in fish

The V regions of channel catfish H chain cDNA clones have been analyzed. Based upon sequence relationships and hybridization analyses, five different groups of VH genes are identified whose definition is consistent with that of five different VH families. Genomic Southern blots indicate that as many as 100 different germ-line VH genes are likely represented by these families. The sequence diversity between identified members of these different families is similar in magnitude to the divergence represented between members of different human or mouse VH families. The FR regions are the most conserved regions when members of different catfish VH families are compared; specific amino acid positions appear to be highly conserved in phylogeny. Equally important is that diversity is represented in complementarity-determining regions CDR1 and CDR2 in members of the different families as well as in members of the same VH family. These results suggest that an extensive repertoire of VH genes can contribute to antibody diversity in this lower vertebrate. Sequence comparisons indicate that one of the catfish VH families shares considerable structural similarity to several higher vertebrate VH gene families--a relationship which suggests that this VH family may be ancestral to some VH gene families of higher vertebrates. Characteristic of the genomic organization of higher vertebrate H chains, catfish appear to have different VH families wherein a VH gene likely undergoes functional recombination with putative DH gene segments and one of apparently several different JH segments. The recombined V region is expressed with the same C region gene. These combined results suggest that bony fishes are the earliest known phylogenetic representatives to have evolved extensive V region gene families.     

Organization and evolution of the rat tyrosine hydroxylase gene

This report describes the organization of the rat tyrosine hydroxylase (TH) gene and compares its structure with the human phenylalanine hydroxylase gene. Both genes are single copy and contain 13 exons separated by 12 introns. Remarkably, the positions of 10 out of 12 intron/exon boundaries are identical for the two genes. These results support the idea that these hydroxylase genes are members of a gene family which has a common evolutionary origin. We predict that this ancestral gene would have encoded exons similar to those of TH prior to evolutionary drift to other members of this gene family.     

Tissue-specific and developmentally regulated expression of a cluster of tandemly arrayed cell wall-associated kinase-like kinase genes in Arabidopsis

The Arabidopsis cell wall-associated kinase (WAK) and WAK-like kinase (WAKL) family of receptor-like kinase genes encodes transmembrane proteins with a cytoplasmic serine/threonine kinase domain and an extracellular region containing epidermal growth factor-like repeats. Previous studies have suggested that some WAK members are involved in plant defense and heavy metal responses, whereas others are required for cell elongation and plant development. The WAK/WAKL gene family consists of 26 members in Arabidopsis and can be divided into four groups. Here, we describe the characterization of group 2 members that are composed of a cluster of seven tandemly arrayed WAKL genes. The predicted WAKL proteins are highly similar in their cytoplasmic region but are more divergent in their predicted extracellular ligand-binding region. WAKL7 encodes a truncated WAKL isoform that is predicted to be secreted from the cytoplasm. Ratios of nonsynonymous to synonymous substitutions suggest that the extracellular region is subject to diversifying selection. Comparison of the WAKL and WAK gene clusters suggests that they arose independently. Protein gel-blot and immunolocalization analyses suggest that WAKL6 is associated with the cell wall. Histochemical analyses of WAKL promoters fused with the beta-glucuronidase reporter gene have shown that the expressions of WAKL members are developmentally regulated and tissue specific. Unlike WAK members whose expressions were found predominately in green tissues, WAKL genes are highly expressed in roots and flowers. The expression of WAKL5 and WAKL7 can be induced by wounding stress and by the salicylic acid analog 2,6-dichloroisonicotinic acid in an nonexpressor of pathogenesis-related gene 1-dependent manner, suggesting that they, like some WAK members, are wound inducible and can be defined as pathogenesis-related genes.     

A tripartite clustering analysis on microrna, gene and disease model

Alteration of gene expression in response to regulatory molecules or mutations could lead to different diseases. MicroRNAs (miRNAs) have been discovered to be involved in regulation of gene expression and a wide variety of diseases. In a tripartite biological network of human miRNAs, their predicted target genes and the diseases caused by altered expressions of these genes, valuable knowledge about the pathogenicity of miRNAs, involved genes and related disease classes can be revealed by co-clustering miRNAs, target genes and diseases simultaneously. Tripartite co-clustering can lead to more informative results than traditional co-clustering with only two kinds of members and pass the hidden relational information along the relation chain by considering multi-type members. Here we report a spectral co-clustering algorithm for k-partite graph to find clusters with heterogeneous members. We use the method to explore the potential relationships among miRNAs, genes and diseases. The clusters obtained from the algorithm have significantly higher density than randomly selected clusters, which means members in the same cluster are more likely to have common connections. Results also show that miRNAs in the same family based on the hairpin sequences tend to belong to the same cluster. We also validate the clustering results by checking the correlation of enriched gene functions and disease classes in the same cluster. Finally, widely studied miR-17-92 and its paralogs are analyzed as a case study to reveal that genes and diseases co-clustered with the miRNAs are in accordance with current research findings.     

Genomic characterization of the human heterotrimeric G protein alpha, beta, and gamma subunit genes.

Heterotrimeric guanine nucleotide binding proteins (G proteins) transduce extracellular signals received by transmembrane receptors to effector proteins. Each subunit of the G protein complex is encoded by a member of one of three corresponding gene families. Currently, 16 different members of the alpha subunit family, 5 different members of the beta subunit family, and 11 different members of the gamma subunit family have been described in mammals. Here we have identified and characterized Bacterial Artificial Chromosomes (BACs) containing the human homologs of each of the alpha, beta, and gamma subunit genes as well as a G alpha11 pseudogene and a previously undiscovered G gamma5-like gene. The gene structure and chromosome location of each gene was determined, as were the orientations of paired genes. These results provide greater insight into the evolution and functional diversity of the mammalian G protein subunit genes.     

Role of gene duplication in evolution

It is now known that many multigene and supergene families exist in eukaryote genomes: multigene families with uniform copy members like genes for ribosomal RNA, those with variable members like immunoglobulin genes, and supergene families such as those for various growth factor and hormone receptors. Many such examples indicate that gene duplication and subsequent differentiation are extremely important for organismal evolution. In particular, gene duplication could well have been the primary mechanism for the evolution of complexity in higher organisms. Population genetic models for the origin of gene families with diverse functions are presented, in which natural selection favors those genomes with more useful mutants in duplicated genes. Since any gene has a certain probability of degenerating by mutation, success versus failure in acquiring a new gene by duplication may be expressed as the ratio of probabilities of spreading of useful versus detrimental mutations in redundant gene copies. Also examined are the effects of gene duplication on evolution by compensatory advantageous mutations. Results of the analyses show that both natural selection and random drift are important for the origin of gene families. In addition, interaction between molecular mechanisms such as unequal crossing-over and gene conversion, and selection or drift is found to have a large effect on evolution by gene duplication.     

Phylogenetic tests of the hypothesis of block duplication of homologous genes on human chromosomes 6, 9, and 1

There are 10 gene families that have members on both human chromosome 6 (6p21.3, the location of the human major histocompatibility complex [MHC]) and human chromosome 9 (mostly 9q33-34). Six of these families also have members on mouse chromosome 17 (the mouse MHC chromosome) and mouse chromosome 2. In addition, four of these families have members on human chromosome 1 (1q21-25 and 1p13), and two of these have members on mouse chromosome 1. One hypothesis to explain these patterns is that members of the 10 gene families of human chromosomes 6 and 9 were duplicated simultaneously as a result of polyploidization or duplication of a chromosome segment ("block duplication"). A subsequent block duplication has been proposed to account for the presence of representatives of four of these families on human chromosome 1. Phylogenetic analyses of the 9 gene families for which data were available decisively rejected the hypothesis of block duplication as an overall explanation of these patterns. Three to five of the genes on human chromosomes 6 and 9 probably duplicated simultaneously early in vertebrate history, prior to the divergence of jawed and jawless vertebrates, and shortly after that, all four of the genes on chromosomes 1 and 9 probably duplicated as a block. However, the other genes duplicated at different times scattered over at least 1.6 billion years. Since the occurrence of these clusters of related genes cannot be explained by block duplication, one alternative explanation is that they cluster together because of shared functional characteristics relating to expression patterns.      

Sequence analysis of members of the human Ig VH4 gene family derived from a single VH locus. Identification of novel germ-line members.

We have generated a mouse x human heterohybridoma that contains a single copy of chromosome 14 and, thus, a haploid set of Ig VH genes. This cell line was used to investigate the germ-line content and nucleotide sequences of members of the VH4 gene family in a polymerase chain reaction-based approach. The analysis of 58 full-length sequences revealed the presence of 12 different germ-line VH4 genes, each of which is potentially functional. These germ-line VH4 genes were compared with the nucleotide sequences of published VH4 genes. Three VH4 genes were 100% identical to previously published sequences and belong to a group of VH4 genes that are strongly conserved and highly prevalent in the human population. Three VH4 genes in our collection displayed greater than 99.3% sequence identity with reported germ-line VH4 sequences and likely represent allelic counterparts of these genes. Six genes displayed less than 97.2% sequence identity with published VH4 genes and were identified as novel members of the human VH4 gene family or more distantly related alleles of known VH4 genes. Collectively, these data suggest that, overall, the human VH4 gene family may be more diverse than hitherto assumed, whereas a number of individual members are nonpolymorphic and extremely well conserved.     

Rhox homeobox gene cluster: recent duplication of three family members

We recently reported the discovery of a homeobox gene cluster on the mouse X chromosome, Rhox, whose 12 members are selectively expressed in specific cell types in reproductive organs. Here we report the existence of 20 additional Rhox homeobox genes in this gene cluster. Most of the newly identified Rhox paralogs retain the same order and relative orientation as three of the originally described Rhox genes, suggesting that they arose from recent duplications of this trimer unit. Many of these new Rhox family members are expressed in the testis and placenta. Analysis of synonymous and nonsynonymous substitutions in their homeodomain region suggests that these new Rhox paralogs duplicated so recently that their encoded proteins have not yet acquired distinct DNA-binding specificities. The existence of these new Rhox genes provides an opportunity to examine the initial stages of gene cluster evolution.     

Cercozoa comprises both EF-1α-containing and EFL-containing members

Elongation factor 1α (EF-1α) and elongation factor-like protein (EFL) are considered to be functionally equivalent proteins involved in peptide synthesis. Eukaryotes can be fundamentally divided into ‘EF-1α-containing’ and ‘EFL-containing’ types. Recently, EF-1α and EFL genes have been surveyed across the diversity of eukaryotes to explore the origin and evolution of EFL genes. Although the phylum Cercozoa is a diverse group, gene data for either EFL or EF-1α are absent from all cercozoans except chlorarachniophytes which were previously defined as EFL-containing members. Our survey revealed that two members of the cercozoan subphylum Filosa (Thaumatomastix sp. and strain YPF610) are EFL-containing members. Importantly, we identified EF-1α genes from two members of Filosa (Paracercomonas marina and Paulinella chromatophora) and a member of the other subphylum Endomyxa (Filoreta japonica). All cercozoan EFL homologues could not be recovered as a monophyletic group in maximum-likelihood and Bayesian analyses, suggesting that lateral gene transfer was involved in the EFL evolution in this protist assemblage. In contrast, EF-1α analysis successfully recovered a monophyly of three homologues sampled from the two cercozoan subphyla. Based on the results, we postulate that cercozoan EF-1α genes have been vertically inherited, and the current EFL-containing species may have secondarily lost their EF-1α genes.     

Genomic organization of hsp90 gene family in Arabidopsis

We have isolated six members of the hsp90 gene family from Arabidopsis thaliana. Three genes designated hsp81.2, 81.3 and 81.4 are clustered within a 15 kb genomic region while two of these are 1.5 kb apart in a head-to-head orientation. The deduced amino acid sequence shows that the members can be divided into two types. The hsp81.1, 81.2, 81.3 and 81.4 genes comprise the cytosolic hsp90 type having few introns. However, the hsp88.1 and 89.1 genes comprising the organelle type are composed of 18 or 19 introns. Sequence comparison showed there is high homology among the cytosolic members while there is less homology among the organelle members. The expression of the hsp90 genes and mRNA accumulation in plants and calli is very low at control temperatures and is strongly induced by heat-shock. Arsenite stress strongly stimulates the expression of this gene family.