Major facilitator superfamily.

The major facilitator superfamily (MFS) is one of the two largest families of membrane transporters found on Earth. It is present ubiquitously in bacteria, archaea, and eukarya and includes members that can function by solute uniport, solute/cation symport, solute/cation antiport and/or solute/solute antiport with inwardly and/or outwardly directed polarity. All homologous MFS protein sequences in the public databases as of January 1997 were identified on the basis of sequence similarity and shown to be homologous. Phylogenetic analyses revealed the occurrence of 17 distinct families within the MFS, each of which generally transports a single class of compounds. Compounds transported by MFS permeases include simple sugars, oligosaccharides, inositols, drugs, amino acids, nucleosides, organophosphate esters, Krebs cycle metabolites, and a large variety of organic and inorganic anions and cations. Protein members of some MFS families are found exclusively in bacteria or in eukaryotes, but others are found in bacteria, archaea, and eukaryotes. All permeases of the MFS possess either 12 or 14 putative or established transmembrane alpha-helical spanners, and evidence is presented substantiating the proposal that an internal tandem gene duplication event gave rise to a primordial MFS protein prior to divergence of the family members. All 17 families are shown to exhibit the common feature of a well-conserved motif present between transmembrane spanners 2 and 3. The analyses reported serve to characterize one of the largest and most diverse families of transport proteins found in living organisms.

Phylogenetic characterization of novel transport protein families revealed by genome analyses.

As a result of recent genome sequencing projects as well as detailed biochemical, molecular genetic and physiological experimentation on representative transport proteins, we have come to realize that all organisms possess an extensive but limited array of transport protein types that allow the uptake of nutrients and excretion of toxic substances. These proteins fall into phylogenetic families that presumably reflect their evolutionary histories. Some of these families are restricted to a single phylogenetic group of organisms and may have arisen recently in evolutionary time while others are found ubiquitously and may be ancient. In this study we conduct systematic phylogenetic analyses of 26 families of transport systems that either had not been characterized previously or were in need of updating. Among the families analyzed are some that are bacterial-specific, others that are eukaryotic-specific, and others that are ubiquitous. They can function by either a channel-type or a carrier-type mechanism, and in the latter case, they are frequently energized by coupling solute transport to the flux of an ion down its electrochemical gradient. We tabulate the currently sequenced members of the 26 families analyzed, describe the properties of these families, and present partial multiple alignments, signature sequences and phylogenetic trees for them all.

Fine-scale genetic mapping of two Pierce's disease resistance loci and a major segregation distortion region on chromosome 14 of grape.

A refined genetic map of chromosome 14, which contains the Pierce's disease (PD) resistance locus, was created from three grape mapping populations. The source of PD resistance in these populations was b43-17, a male form of Vitis arizonica Engelm. that is homozygous resistant. The resistance locus segregated as a single dominant gene and mapped as PdR1a in the F1 selection F8909-17 (9621 population) and as PdR1b in a sibling F1 selection F8909-08 (04190 population). These two full sibs inherited either allele of the Pierce's disease resistance locus from the b43-17 parent, which is homozygous at that locus. The 9621 population consisted of 425 progeny and PdR1a mapped between markers VvCh14-56/VvCh14-02 and UDV095 within a 0.6 cM genetic distance. The 04190 population consisted of 361 progeny and PdR1b mapped between markers VvCh14-02 and UDV095/VvCh14-10 within a 0.4 cM distance. Many of the markers present on chromosome 14 were distorted with an excess of female alleles in the 04190 and 04373 population (developed from a cross of V. vinifera L. F2-35 x b43-17) indicating that potential gametophytic factors are present in this region. Common markers from this region within the 9621 population were not distorted except Scu15. When these markers were compared to V. vinifera-based maps of chromosome 14 they were also distorted suggesting the involvement of gametophytic factors, and prompting the identification of this region as Vitis-segregation distortion region 1 (V-SDR1). The refined genetic maps developed from this study can be used to identify and clone genes that confer resistance to Pierce's disease.

Evolutionary origins of multidrug and drug-specific efflux pumps in bacteria.

The available genomic sequences of three pathogenic and three nonpathogenic bacteria were analyzed to identify known and putative drug-specific and multidrug resistance transport systems. Escherichia coli was found to encode 29 such pumps, and with the exception of the archaebacterium Methanococcus jannaschii, the numbers of multidrug efflux pumps encoded within genomes of the other organisms were found to be approximately proportional to their total numbers of encoded transport systems as well as to total genome size. The similar numbers of chromosomally encoded multidrug efflux systems in pathogens and nonpathogens suggests that these transporters have not arisen recently in pathogens in response to antimicrobial chemotherapy. Phylogenetic analyses of the four transporter families that contain drug efflux permeases indicate that drug resistance arose rarely during the evolution of each family and that the diversity of current drug efflux pumps within each family arose from just one or a very few primordial systems. However, although the ability to confer drug efflux appears to have emerged on only a few occasions in evolutionary time and was stably maintained as an evolutionary trait, modulation of the substrate specificities of these systems has occurred repeatedly. A speculative model is presented that may explain the apparent capability of these multidrug transport systems to mediate drug transport from the cytoplasm or directly from the phospholipid bilayer.

The major facilitator superfamily.

In 1998 we updated earlier descriptions of the largest family of secondary transport carriers found in living organisms, the major facilitator superfamily (MFS). Seventeen families of transport proteins were shown to comprise this superfamily. We here report expansion of the MFS to include 29 established families as well as five probable families. Structural, functional, and mechanistic features of the constituent permeases are described, and each newly identified family is shown to exhibit specificity for a single class of substrates. Phylogenetic analyses define the evolutionary relationships of the members of each family to each other, and multiple alignments allow definition of family-specific signature sequences as well as all well-conserved sequence motifs. The work described serves to update previous publications and allows extrapolation of structural, functional and mechanistic information obtained with any one member of the superfamily to other members with limitations determined by the degrees of sequence divergence.