Puzzle of crystallin diversity in eye lenses

Moonlighting enzymes: when cellular context defines specificity

It is not often realized that the absolute protein specificity is an exception rather than a rule. Two major kinds of protein multi-specificities are promiscuity and moonlighting. This review discusses the idea of enzyme specificity and then focusses on moonlighting. Some important examples of protein moonlighting, such as crystallins, ceruloplasmin, metallothioniens, macrophage migration inhibitory factor, and enzymes of carbohydrate metabolism are discussed. How protein plasticity and intrinsic disorder enable the removing the distinction between enzymes and other biologically active proteins are outlined. Finally, information on important roles of moonlighting in human diseases is updated.

Co-option of stress mechanisms in the origin of evolutionary novelties

It is widely accepted that stressful conditions can facilitate evolutionary change. The mechanisms elucidated thus far accomplish this with a generic increase in heritable variation that facilitates more rapid adaptive evolution, often via plastic modifications of existing characters. Through scrutiny of different meanings of stress in biological research, and an explicit recognition that stressors must be characterized relative to their effect on capacities for maintaining functional integrity, we distinguish between: (1) previously identified stress‐responsive mechanisms that facilitate evolution by maintaining an adaptive fit with the environment, and (2) the co‐option of stress‐responsive mechanisms that are specific to stressors leading to the origin of novelties via compensation. Unlike standard accounts of gene co‐option that identify component sources of evolutionary change, our model documents the cost‐benefit trade‐offs and thereby explains how one mechanism—an immediate response to acute stress—is transformed evolutionarily into another—routine protection from recurring stressors. We illustrate our argument with examples from cell type origination as well as processes and structures at higher levels of organization. These examples suggest a general principle of evolutionary origination based on the capacity to switch between regulatory states related to reproduction and proliferation versus survival and differentiation.

Diverse Evolutionary Origins and Mechanisms of Lens Regeneration

In this review, we compare and contrast the three different forms of vertebrate lens regeneration: Wolffian lens regeneration, cornea-lens regeneration, and lens regeneration from lens epithelial cells. An examination of the diverse cellular origins of these lenses, their unique phylogenetic distribution, and the underlying molecular mechanisms, suggests that these different forms of lens regeneration evolved independently and utilize neither conserved nor convergent mechanisms to regulate these processes.

Dynamic reorganization of metabolic enzymes into intracellular bodies

Both focused and large-scale cell biological and biochemical studies have revealed that hundreds of metabolic enzymes across diverse organisms form large intracellular bodies. These proteinaceous bodies range in form from fibers and intracellular foci--such as those formed by enzymes of nitrogen and carbon utilization and of nucleotide biosynthesis--to high-density packings inside bacterial microcompartments and eukaryotic microbodies. Although many enzymes clearly form functional mega-assemblies, it is not yet clear for many recently discovered cases whether they represent functional entities, storage bodies, or aggregates. In this article, we survey intracellular protein bodies formed by metabolic enzymes, asking when and why such bodies form and what their formation implies for the functionality--and dysfunctionality--of the enzymes that comprise them. The panoply of intracellular protein bodies also raises interesting questions regarding their evolution and maintenance within cells. We speculate on models for how such structures form in the first place and why they may be inevitable.

A new splice variant of the glutamate-aspartate transporter: cloning and immunolocalization of GLAST1c in rat, pig and human brains

GLAST (EAAT1) is an abundant glial glutamate transporter in the mammalian brain. It plays important roles in terminating excitatory transmission in grey matter, as well as pathophysiological roles, including protecting white matter from excitotoxic injury. In normal brain, alternative splicing of GLAST has been described: GLAST1a and GLAST1b arise from the splicing out of exons 3 and 9, respectively. This study describes the isolation of a novel cDNA clone from neonatal hypoxic pig brain, referred to as GLAST1c, where exons 5 and 6 are skipped. GLAST1c encodes a protein of 430 amino acids. RT-PCR analysis showed that GLAST1c mRNA was readily detectable in control and hypoxic pig cortex, as well as in various brain regions of rat (cortex, mid, hind and cerebellum), and human cortex, retina and optic nerve. We have raised antibodies that selectively recognize GLAST1c and demonstrate expression of this novel splice variant in astrocytes and oligodendrocytes in rat brain, pig brain and human brain, including grey and white matter. Similarly expression of GLAST1c was observed in primary astrocyte cultures and in cultured oligodendrocytes. In unstimulated astrocytes GLAST1c exhibited an intracellular peri-nuclear distribution similar to that observed when GFP-tagged GLAST1c was transfected into COS 7 cells. In astrocytes this protein rapidly redistributed to the surface upon stimulation of protein kinase with phorbol esters. We conclude that GLAST1c may represent an astrocyte and oligodendrocyte glutamate transporter, though this could not be formally validated by D-aspartate uptake studies, due to the low transfection efficiency of constructs into COS 7 cells.

Self-assembling enzymes and the origins of the cytoskeleton

The bacterial cytoskeleton is composed of a complex and diverse group of proteins that self-assemble into linear filaments. These filaments support and organize cellular architecture and provide a dynamic network controlling transport and localization within the cell. Here, we review recent discoveries related to a newly appreciated class of self-assembling proteins that expand our view of the bacterial cytoskeleton and provide potential explanations for its evolutionary origins. Specifically, several types of metabolic enzymes can form structures similar to established cytoskeletal filaments and, in some cases, these structures have been repurposed for structural uses independent of their normal roles. The behaviors of these enzymes suggest that some modern cytoskeletal proteins may have evolved from dual-role proteins with catalytic and structural functions.

Recapitulation of human betaB1-crystallin promoter activity in transgenic zebrafish

Development of the eye is morphologically similar among vertebrates, indicating that the underlying mechanism regulating the process may have been highly conserved during evolution. Herein we analyzed the promoter of the human betaB1-crytallin gene in zebrafish by transgenic experiments. To delineate the evolutionarily conserved regulatory elements, we performed serial deletion assays in the promoter region. The results demonstrated that the -90/+61-bp upstream proximal promoter region is sufficient to confer lens-tissue specificity to the human betaB1-crystallin gene in transgenic zebrafish. Through phylogenetic sequence comparisons and an electrophoretic mobility shift assay (EMSA), a highly conserved cis-element of a six-base pair sequence TG(A/C)TGA, the consensus sequence for the Maf protein binding site, within the proximal promoter region was revealed. Further, a site-mutational assay showed that this element is crucial for promoter activity. These data suggest that the fundamental transcriptional regulatory mechanism of the betaB1-crystallin gene has been well conserved between humans and zebrafish, and plausibly among all vertebrates, during evolution.

Functional analysis of the chicken delta1-crystallin enhancer activity in Drosophila reveals remarkable evolutionary conservation between chicken and fly

Functional conservation of enhancers among evolutionarily diverged organisms is a powerful way to identify basic regulatory circuits and key developmental regulators. This is especially applicable to Crystallin genes. Despite unexpected heterogeneity and diversity in their DNA sequences, many studies have revealed that most of the Crystallin genes are regulated by a relatively small set of developmentally important transcription factors. The chicken delta1-crystallin is one of the best-characterized Crystallin genes. Its lens-specific regulation is governed by a 30 bp long DC5 fragment present in the third intron of the gene. DC5 contains PAX6 and SOX2 binding sites, and its activity depends on the cooperative binding of these two transcription factors. To test the idea that Pax6 and Sox2, together with the DC5 enhancer, could form a basic regulatory circuit functional in distantly related animals, we introduced the DC5 fragment into Drosophila and studied its activation pattern and regulation. The results show that the DC5 enhancer is not only active in the compound eye but, remarkably, is specifically active in those cells responsible for Crystallin secretion in Drosophila, i.e. the cone cells. However, regulation of the DC5 enhancer is carried out not by Pax6, but by Pax2 (D-Pax2; shaven--FlyBase) in combination with the Sox2 homologue SoxN. Both proteins (D-PAX2 and SOXN) bind cooperatively to the DC5 fragment and activate the enhancer synergistically. As PAX6 and PAX2 proteins derive from the same ancestor, we propose that during evolution Pax6 function in vertebrate lens development was retained by Pax2 in Drosophila.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

Effect of exogenous stress on the tissue-cultured mouse lens epithelial cells

The effects of heat, oxidative and osmotic stress on heat shock proteins (HSP-70(I), HSC-70, and HSP-40 of tissue cultured mouse lens epithelial cells (alphaTN-4) were investigated. The effect of stress on the heat shock transcription factor (HSF-1), and a nuclear matrix protein (NM-60) of alphaTN-4 cells was also examined. Cells were exposed to heat (45 degrees C), oxidative stress (50 mM H(2)0(2)) and osmotic (600 mM medium) shock for 30 min, and then allowed to recover for 18 h in physiological medium. Control cells were maintained at 37 degrees C in an isosmolar medium. Cellular proteins were isolated and fractionated by SDS-PAGE. Western blot was used to determine the levels of HSP and nuclear proteins. Heat stressed cells were also examined, by immunofluorescence, for the specific localization of NM-60 and HSF-1. The results revealed that both NM-60 and HSF-I were present in specific locations in normal and heat-stressed cell nuclei. Nuclei isolated immediately after stress showed localization of fluorescence near the nuclear membrane. When heat stressed cells were allowed to recuperate at 37 degrees C, most of the fluorescence were relocated in discrete areas of the nucleus. These experiments showed that alphaTN-4 cells responded to stress by overexpression of HSP-70(I) and HSP-40. This increase was not present immediately after the end of the stress period, but clearly evident at 18 h of recovery in physiological medium. Immunofluorescent data suggest that heat stress induced the localization of NM-60 and HSF-1 near the nuclear membrane.

The cell adhesion molecule retina cognin is a cell surface protein disulfide isomerase that uses disulfide exchange activity to modulate cell adhesion

The retina cell adhesion molecule, R-cognin, shares cDNA sequence with protein disulfide isomerase (PDI) but has a different molecular size and subcellular location. We asked whether R-cognin originated from a unique PDI gene transcript or was a product of posttranscriptional processing. The 3'-terminal partial cDNA clone for R-cognin was extended by both 5' RACE and by PCR from sequence near the 5' end of the PDI-translated region. The cDNA sequence was compared to those of chicken, bovine, and human PDI. The R-cognin cDNA sequence was identical to that of chicken PDI and differed by less than 10% from mammalian PDI proteins. The role of the disulfide exchange activity characteristic of both proteins was studied by assessing the cell-aggregation-enhancing ability and tissue specificity of R-cognin and recombinant human PDI and its derivatives. Chicken and normal human PDI proteins showed tissue- and developmental-specific enhancement of cell aggregation identical to R-cognin, and this activity was blocked by inactivation of the -WCGHC- motifs which function in disulfide exchange. Dependence of retina cell aggregation on disulfide exchange activity was shown by blocking that activity with the inhibitor, DTNB, or with a recombinant human PDI with the -WCGHC- motif cysteines mutated. The results suggest that one -WCGHC- motif in R-cognin is sufficient and that the more C-terminal motif is most active. We conclude that R-cognin is a tissue-specific protein product of the standard PDI chicken gene. The -WCGHC- motif in mature R-cognin is necessary, but not sufficient, for cell adhesion.

Expression and functional characteristics of calpain 3 isoforms generated through tissue-specific transcriptional and posttranscriptional events

Calpain 3 is a nonlysosomal cysteine protease whose biological functions remain unknown. We previously demonstrated that this protease is altered in limb girdle muscular dystrophy type 2A patients. Preliminary observations suggested that its gene is subjected to alternative splicing. In this paper, we characterize transcriptional and posttranscriptional events leading to alterations involving the NS, IS1, and IS2 regions and/or the calcium binding domains of the mouse calpain 3 gene (capn3). These events can be divided into three groups: (i) splicing of exons that preserve the translation frame, (ii) inclusion of two distinct intronic sequences between exons 16 and 17 that disrupt the frame and would lead, if translated, to a truncated protein lacking domain IV, and (iii) use of an alternative first exon specific to lens tissue. In addition, expression of these isoforms seems to be regulated. Investigation of the proteolytic activities and titin binding abilities of the translation products of some of these isoforms clearly indicated that removal of these different protein segments affects differentially the biochemical properties examined. In particular, removal of exon 6 impaired the autolytic but not fodrinolytic activity and loss of exon 16 led to an increased titin binding and a loss of fodrinolytic activity. These results are likely to impact our understanding of the pathophysiology of calpainopathies and the development of therapeutic strategies.

Involvement of retinoic acid/retinoid receptors in the regulation of murine alphaB-crystallin/small heat shock protein gene expression in the lens

Crystallins are a diverse group of abundant soluble proteins that are responsible for the refractive properties of the transparent eye lens. We showed previously that Pax-6 can activate the alphaB-crystallin/small heat shock protein promoter via the lens-specific regulatory regions LSR1 (-147/-118) and LSR2 (-78/-46). Here we demonstrate that retinoic acid can induce the accumulation of alphaB-crystallin in N/N1003A lens cells and that retinoic acid receptor heterodimers (retinoic acid receptor/retinoid X receptor; RAR/RXR) can transactivate LSR1 and LSR2 in cotransfection experiments. DNase I footprinting experiments demonstrated that purified RAR/RXR heterodimers will occupy sequences resembling retinoic acid response elements within LSR1 and LSR2. Electrophoretic mobility shift assays using antibodies indicated that LSR1 and LSR2 can interact with endogenous RAR/RXR complexes in extracts of cultured lens cells. Pax-6 and RAR/RXR together had an additive effect on the activation of alphaB-promoter in the transfected lens cells. Thus, the alphaB-crystallin gene is activated by Pax-6 and retinoic acid receptors, making these transcription factors examples of proteins that have critical roles in early development as well as in the expression of proteins characterizing terminal differentiation.

The mouse transketolase (TKT) gene: cloning, characterization, and functional promoter analysis

The transketolase (TKT) gene is expressed 30-50 times more highly in the mature mouse cornea than in other tissues. Here, we have cloned and characterized the 30- to 40-kb single-copy mouse TKT gene. Sequence analysis supports the suggestion that present-day TKT and TKT-like genes arose from the duplication of a single common ancestral gene. A 6-bp polymorphism is present between different mouse strains in the noncoding region of exon 2. 5' RACE and primer extension analyses indicated that two regions separated by 630 bp are used as transcription initiation sites; both mRNAs appear to use a common initiator ATG codon. The minor distal transcription initiation site, preceded by a TATA sequence, is utilized in liver and is followed by an untranslated exon (exon 1). The major proximal transcription initiation site lies within intron 1, is used in cornea and liver, lacks a TATA sequence, is GC rich, and initiates at multiple sites within a 10-bp span, resembling the promoters of other housekeeping genes. In transfected cornea and lens cell lines, the -49/+90 fragment fused to the CAT gene acted as a minimal promoter, with higher activity noted for the -510/+91 fragment. TKT mRNA levels increased sixfold in the mouse cornea in vivo within 1-2 days of eye opening and were elevated in a lens cell line exposed to H2O2 or the glutathione-specific oxidizing agent diamide and in whole newborn mouse eyes incubated in the presence of light, consistent with multiple consensus stress-inducible control sequences in the TKT promoter regions. Taken together, these observations suggest that oxidative stress may play a role in the regulation of this gene in the cornea.

Transcriptional regulation of the mouse alpha A-crystallin gene: binding of USF to the -7/+5 region

Lens preferred-expression of the mouse alpha A-crystallin gene (alpha A-cry) is regulated at the transcriptional level by multiple elements located in the 5' flanking region of the gene. Here we present the first analysis of the functional role of the mouse alpha A-cry +1 region and the protein(s) which bind to it. The -7/+5 region of this promoter exhibits sequence similarity with the consensus upstream stimulating factor (USF) transcription factor binding site. A wild type oligodeoxyribonucleotide (oligo) spanning the mouse alpha A-cry -15/+15 region specifically inhibited the activity of a mouse alpha A-cry promoter-cat gene fusion (p alpha A 111aCAT) in competitive co-transfection studies in the mouse alpha TN4-1 lens cell line, as did an oligo containing the adenovirus 2 major late promoter strong USF binding site. In contrast, an alpha A-cry oligo mutated (-3/+3) within the USF-like binding site did not inhibit p alpha A111aCAT activity. Western blot analysis indicated that alpha TN4-1 cells express USF1. Co-transfection of p alpha A111aCAT and a USF1 cDNA expression vector into alpha TN4-1 cells resulted in a repression of mouse alpha A-cry promoter activity. Electrophoretic mobility shift analyses (EMSA) demonstrated that proteins in an alpha TN4-1 nuclear extract form a single major complex on synthetic oligos spanning the mouse alpha A-cry -15/+15 region. The formation of this complex was inhibited by the presence of unlabeled -15/+15 oligos or an anti-USF1 antibody. In addition, purified USF1 bound to this region, producing a complex similar in size to that observed with alpha TN4-1 nuclear extracts. Taken together, our findings show that USF can bind to the mouse alpha A-cry +1 site, and support the possibility that USF plays a role in promoter activity of this gene. Sequence similarities surrounding the +1 region of the alpha A-cry gene of the mouse, mole rat, hamster, and human, as well as the previously observed utilization of USF by different cry promoters suggest that USF contributes to the high expression of many crys in the ocular lens of diverse species.

Temporospatial expression of the small HSP/alpha B-crystallin in cardiac and skeletal muscle during mouse development

Although the small (22 Kd) heat shock protein/alpha B-crystallin functions as a major structural protein and molecular chaperone in the vertebrate lens, little is known about the protein's role in nonlenticular tissues such as the heart and skeletal muscle. Recent studies have demonstrated that alpha B-crystallin expression is uniquely regulated during myogenesis in vitro. We report here for the first time that the temporal and spatial expression of alpha B-crystallin is similarly regulated in vivo during mouse embryogenesis. Expression of alpha B-crystallin mRNA was detected by in situ hybridization in the primitive heart at 8.5 days postconception (p.c.) and in the myotome of the somites at 10.5 days p.c. This tissue-restricted pattern was corroborated by immunohistochemical studies. alpha B-crystallin mRNA and protein expression were uniform in the developing atria and ventricles without regional differences or gradients. alpha B-crystallin expression was absent in the endocardial cushion, pulmonary trunk, aorta, and endothelium. Examination of muscle precursors revealed expression throughout the dorsoventral aspect of the myotomes and in developing skeletal muscle. Our findings suggest that alpha B-crystallin may serve pivotal roles as a structural protein and a molecular chaperone in myofiber stabilization of metabolically active tissues during early embryogenesis. Thus, early alpha B-crystallin expression in myogenic lineages supports the hypothesis that the putative functions of alpha B-crystallin are coupled to the activation of genetic programs responsible for myogenic differentiation and cardiac morphogenesis.

Transketolase is a major protein in the mouse cornea

Earlier experiments in this laboratory identified a highly expressed 65-68-kDa protein in both mouse and human corneas (Cuthbertson, R. A. , Tomarev, S. I., and Piatigorsky J. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 4004-4008). Here, we demonstrate that this protein is transketolase (TKT; EC 2.2.1.1), an enzyme in the nonoxidative branch of the pentose-phosphate pathway, based on peptide and cDNA isolation and sequence analysis of mouse cornea protein and RNA samples, respectively. While expressed at low levels in a number of tissues, the 2.1-kilobase TKT mRNA was expressed at a 50-fold higher level in the adult mouse cornea. The area of most abundant expression was localized to the cornea epithelial cell layer by in situ hybridization. Western blot analysis confirmed TKT protein abundance in the cornea and indicated that TKT may comprise as much as 10% of the total soluble protein of the adult mouse cornea. Soluble cornea extracts exhibited a correspondingly high level of TKT enzymatic activity. TKT expression increased progressively through cornea maturation, as shown by Northern blot, in situ hybridization, Western blot, and enzymatic analyses. TKT mRNA and protein were expressed at low levels in the cornea prior to eye opening, while markedly increased levels were observed after eye opening. Taken together, these observations suggest that TKT may be a cornea enzyme-crystallin, and suggest that the crystallin paradigm and concept of gene sharing, once thought to be restricted to the lens, apply to other transparent ocular tissues.

Cellular and molecular features of lens differentiation: a review of recent advances

In this paper, the more recent literature pertaining to differentiation in the developing vertebrate lens is reviewed in relation to previous work. The literature reviewed reveals that the developing lens has been, and will continue to be, a useful model system for the examination of many fundamental processes occurring during embryonic development. Areas of lens development reviewed here include: the induction and early embryology of the lens; lens cell culture techniques; the role of growth factors and cytokines; the involvement of gap junctions in lens cell-cell communication; the role of cell adhesion molecules, integrins, and the extracellular matrix; the role of the cytoskeleton; the processes of programmed cell death (apoptosis) and lens fibre cell denucleation; the involvement of Pax and Homeobox genes; and crystallin gene regulation. Finally, some speculation is provided as to possible directions for further research in lens development.

Transcriptional regulation of genes for ornithine cycle enzymes

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Tissue-nonspecific alkaline phosphatase participates in the establishment and growth of feather germs in embryonic chick skin cultures

Alkaline phosphatase activity is present in the mesoderm of embryonic chick skin and becomes spatially restricted to the dermal condensation of the developing feather germs. Inhibitors to tissue-nonspecific (liver/bone/kidney), but not intestinal alkaline phosphatase inhibit the establishment and growth of feather germs in cultured skins. A window of maximum sensitivity to the inhibitor was observed to be the first day of culture when early development and establishment of pattern takes place. The cDNA for the avian tissue-nonspecific alkaline phosphatase was cloned and sequenced, and Southern analysis revealed a single copy of this gene in the avian genome. Northern analysis revealed that a 2.8 kb transcript for this form of alkaline phosphatase is present in developing skin.

Pax-6 and lens-specific transcription of the chicken delta 1-crystallin gene

The abundance of delta-crystallin in the chicken eye lens provides an advantageous marker for tissue-specific gene expression during cellular differentiation. The lens-specific expression of the delta 1-crystallin gene is governed by an enhancer in the third intron, which binds a positive (delta EF2) and negative (delta EF1) factor in its core region. Here we show by DNase I footprinting, electrophoretic mobility-shift assays, and cotransfection experiments with the delta 1-promoter/enhancer fused to the chloramphenicol acetyltransferase reporter gene that the delta 1-crystallin enhancer has two adjacent functional Pax-6 binding sites. We also demonstrate by DNase I footprinting that the delta EF1 site can bind the transcription factor USF, raising the possibility that USF may cooperate with Pax-6 in activation of the chicken delta 1- and alpha A-crystallin genes. These data, coupled with our recent demonstration that Pax-6 activates the alpha A-crystallin gene, suggest that Pax-6 may have been used extensively throughout evolution to recruit and express crystallin genes in the lens.

Unexpected sequence similarity between nucleosidases and phosphoribosyltransferases of different specificity

Amino acid sequences of enzymes that catalyze hydrolysis or phosphorolysis of the N-glycosidic bond in nucleosides and nucleotides (nucleosidases and phosphoribosyltransferases) were explored using computer methods for database similarity search and multiple alignment. Two new families, each including bacterial and eukaryotic enzymes, were identified. Family I consists of Escherichia coli AMP hydrolase (Amn), uridine phosphorylase (Udp), purine phosphorylase (DeoD), uncharacterized proteins from E. coli and Bacteroides uniformis, and, unexpectedly, a group of plant stress-inducible proteins. It is hypothesized that these plant proteins have evolved from nucleosidases and may possess nucleosidase activity. The proteins in this new family contain 3 conserved motifs, one of which was found also in eukaryotic purine nucleosidases, where it corresponds to the nucleoside-binding site. Family II is comprised of bacterial and eukaryotic thymidine phosphorylases and anthranilate phosphoribosyltransferases, the relationship between which has not been suspected previously. Based on the known tertiary structure of E. coli thymidine phosphorylase, structural interpretation was given to the sequence conservation in this family. The highest conservation is observed in the N-terminal alpha-helical domain, whose exact function is not known. Parts of the conserved active site of thymidine phosphorylases and anthranilate phosphoribosyltransferases were delineated. A motif in the putative phosphate-binding site is conserved in family II and in other phosphoribosyltransferases. Our analysis suggests that certain enzymes of very similar specificity, e.g., uridine and thymidine phosphorylases, could have evolved independently. In contrast, enzymes catalyzing such different reactions as AMP hydrolysis and uridine phosphorolysis or thymidine phosphorolysis and phosphoribosyl anthranilate synthesis are likely to have evolved from common ancestors.