Kirk DL
The genetic
program for germ-soma differentiation in Volvox
ANNU REV GENET 31: 359-380 1997
Abstract:
Volvox carteri possesses only two cell types: mortal somatic cells and
potentially immortal asexual reproductive cells called gonidia. Mutational
analysis indicates that three categories of genes play central roles in
programming this germ-soma division of labor: First the gls genes function
during embryogenesis to cause asymmetric divisions that produce large and small
cells. Then the lag genes act in the large cells (gonidial initials) to repress
functions required for somatic development while the regA locus acts in the
small cells (somatic initials) to repress functions required for reproductive
development. Transposon tagging and DNA transformation have recently been used
to recover and characterize the glsA and regA genes, and the sequences of these
genes lead to testable hypotheses about how they play their roles in germ-soma
differentiation.
Kroger N, Lehmann G, Rachel R, et al.
Characterization
of a 200-kDa diatom protein that is specifically associated with a silica-based
substructure of the cell wall
EUR J BIOCHEM 250 (1): 99-105
Abstract:
The cell wall of a diatom is made up of a silica-based scaffold and organic
macromolecules. Proteins located in the cell wall are believed to control
morphogenesis of the species-specific silica structures of the scaffold.
However, data that correlate distinct silica elements and specific proteins
within the diatom cell wall have not been reported. Here, the cell wall protein
HEP2OO (200-kDa HE-extractable protein) from the diatom Cylindrotheca
fusiformis is identified and characterized. HEP200 is tightly associated with a
substructure of the silica scaffold. It is a member of a new protein family, of
which two more members are identified. Each member displays the same bipartite
structure. The N-terminal part consists of a variable number of a repeated
sequence motif (PSCD domain), whereas the C-terminal part is unique,
Immunolocalization experiments revealed the arrangement of different proteins
within the cell wall. Frustulins, a previously described group of
glycoproteins, constitute the outer coat of the cell wall and exhibit a
ubiquitous distribution. In contrast, HEP200 is specifically located at a
subset of about six silica strips in intact cell walls, shielded by frustulins.
This study therefore identifies a diatom cell wall protein (HEP200) that is
associated with a distinct substructure of the silica scaffold.
Kurvari V
Cell wall
biogenesis in Chlamydomonas: molecular characterization of a novel protein
whose expression is up-regulated during matrix formation
MOL GEN GENET 256 (5): 572-580 NOV 1997
Abstract:
In the unicellular eukaryote Chlamydomonas, disruption of cell-matrix
interactions by treatment with a periplasmic matrix metalloproteinase, g-lysin,
activates a signal transduction pathway that results in the rapid synthesis and
secretion of matrix molecules, followed by their assembly into a new matrix. I
have identified and partially characterized several cDNA clones for transcripts
that are dramatically up-regulated following treatment of cells with g-lysin.
Here I report the complete nucleotide sequence and preliminary characterization
of a matrix-related molecule termed Mrp47. The cDNA clone for Mrp47 contained
an insert of 2.5 kb, corresponding to a transcript of 3.0 kb that is encoded by
a single-copy gene. Sequence analysis indicated that Mrp47 cDNA contains an
open reading frame (ORF) that encodes a 46-kDa polypeptide. The putative
polypeptide is unusually rich in the amino acids proline, alanine and serine,
with prolines clustered together in a 30-amino acid N-terminal region and a
80-amino acid C-terminal region. Further analysis of the predicted amino acid
sequence suggested that Mrp47 is likely to be a secreted glycoprotein. Southern
hybridization analysis indicated that Mrp47 is encoded by a single-copy gene in
the Chlamydomonas genome. Database searches suggested that Mrp47 shows homology
to other proline-rich proteins including a surface glycoprotein in Volvox
and verprolin from yeast.
Hoops HJ
Motility in
the colonial and multicellular Volvocales: structure, function, and evolution
PROTOPLASMA 199 (3-4): 99-112 1997
Abstract:
The colonial Volvocales are often said to be composed of Chlamydomonas-like
cells, but there are substantial differences in motility and flagellar
apparatus construction between the unicellular forms and the individual members
of a colony or spheroid. These changes appear to be required for effective
organismal motion and might possibly limit the rate at which new colonial forms
evolve from unicellular ones. The flagellar-beat envelopes in colonial members
are modified such that they beat in the same direction and in parallel planes
with their effective strokes at right angles to the cellular anterior-posterior
axis. These changes result from a series of developmental events of the
flagellar apparatus of the colonial forms while the colony is still an embryo.
Differences in the flagellar-apparatus structure in the members of the
Goniaceae and Volvocaceae are not obviously correlated with the traditional
placement of these algae in a simple volvocine lineage. Effective colonial
motion clearly requires precise positioning and rotational orientation of the
cells within the colony. Almost any arrangement where the cells are placed with
rotational symmetry within the colony results in colonial progression with
rotation. Such rotational symmetry is present from the time of embryogenesis.
The mechanism that leads to organismal steering in behavioral responses (e.g.,
phototaxis) must likewise differ between colonial and unicellular forms. Ln at
least some cases, this appears to result from changes in beat frequency in some
parts of the spheroid, but changes in beat direction cannot be ruled out for
all forms.
Hegemann P
Vision in
microalgae
PLANTA 203 (3): 265-274 NOV 1997
Abstract:
Flagellate green algae such as Chlamydomonas and related genera are guided by
their eyes to places where light conditions are optimal for photosynthetic
growth. These eyes constitute the simplest and most common visual system found
in nature. The eyes contain optics, photoreceptors and the elementary
components of a signal-transduction chain. Rhodopsin serves as the
photoreceptor, as it does in animal vision. Upon light stimulation, its
all-trans-retinal chromophore isomerizes into 13-cis and activates a
photoreceptor channel which leads to a rapid Ca2+ influx into the eyespot
region. At low light levels, the depolarization activates small flagellar
currents which induce in both flagella small but slightly different beating
changes resulting in distinct directional changes. In continuous light, Ca2+
fluxes serve as the molecular basis for phototaxis. In response to flashes of
higher energy the larger photoreceptor currents trigger a massive Ca2+ influx
into the flagella which causes the well-known phobic response. The
identification of proteins contributing to this signalling system has just
begun with the isolation and cloning of the opsins from Chlamydomonas and Volvox.
These plant opsins are highly charged, are not typical seven-helix receptors,
and are believed to form a protein complex with the photoreceptor channel. In
Spermatozopsis, a G-protein has been found which interacts either directly with
the rhodopsin or with the rhodopsin-ion channel complex. By using insertional
mutagenesis, genes coding for proteins that are involved in signalling have
been tagged. One of them is connected to the flagellar channel and crucial for
the flagellar action potential. Elucidation of photoreception in flagellated
algae will provide deeper insight into the development of visual systems,
starting from single-celled organisms and moving up through higher animals.
Nozaki H, Ito M, Uchida H, et al.
Phylogenetic
analysis of Eudorina species (Valvocaceae, Chlorophyta) based on rbcL gene
sequences
J PHYCOL 33 (5): 859-863 OCT 1997
Abstract:
Species and varieties in the genus Eudorina Ehrenberg (Volvocaceae,
Chlorophyta) were evaluated on the basis of phylogenetic analyses of the large
subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) gene sequence
from 14 strains of four Eudorina species, as well as from nine species of
Pleodorina and Volvox. The sequence data suggested that 10 of the 14
Eudorina strains form three separate and robust monophyletic groups within the
nonmonophyletic genus Eudorina. The first group comprises ail three strains
off. unicocca G. M. Smith; the second group consists of one of the E, elegans
Ehrenberg var elegans strains, the E. cylindrica Korshikov strain, and both E.
illinoisensis (Kofoid) Pascher strains; and the third group consists of two
monoecious varieties off, elegans [two strains of E. elegans var synoica
Goldstein and one strain of E. elegans var. carteri (G. hi. Smith) Goldstein].
In addition, E. illinoisensis represents a poly- or paraphyletic species within
the second group. The remaining four strains, all of which are assigned to E.
elegans var. elegans, are nonmonophyletic. Although their position in the
phylogenetic trees is more or less ambiguous, they are ancestral to other taxa
ill the large anisogamous/oogamous monophyletic group including Eudorina,
Pleodorina, and Volvox (except for sect. Volvox). Thus, the four
Eudorina groups resolved in the present molecular phylogeny do not correspond
with the species concepts of Eudorina based on vegetative morphology, but they
do reflect the results Of the previous intercrossing experiments and modes of
monoecious and dioecious sexual reproduction.
Dierks T, Schmidt B, VonFigura K
Conversion of
cysteine to formylglycine: A protein modification in the endoplasmic reticulum
P NATL ACAD SCI
Abstract:
In sulfatases a C-alpha-formylglycine residue is found at a position where
their cDNA sequences predict a cysteine residue. In multiple sulfatase
deficiency, an inherited lysosomal storage disorder, catalytically inactive
sulfatases are synthesized which retain the cysteine residue, indicating that
the C,-formylglycine residue is required for sulfatase activity. Using in vitro
translation in the absence or presence of transport competent microsomes we
found that newly synthesized sulfatase polypeptides carry a cysteine residue and
that the oxidation of its thiol group to an aldehyde is catalyzed in the
endoplasmic reticulum. A linear sequence of 16 residues surrounding the Cys-69
in arylsulfatase A is sufficient to direct the oxidation, This novel protein
modification occurs after or at a late stage of cotranslational protein
translocation into the endoplasmic reticulum when the polypeptide is not yet
folded to its native structure.
Hallmann A, Rappel A, Sumper M
Gene
replacement by homologous recombination in the multicellular green alga Volvox carteri
P NATL ACAD SCI
Abstract:
With only two different cell types, the haploid green alga Volvox represents
the simplest multicellular model system. To facilitate genetic investigations
in this organism, the occurrence of homologous recombination events was
investigated with the intent of developing methods for gene replacement and
gene disruption. First, homologous recombination between two plasmids was
demonstrated by using overlapping nonfunctional fragments of a recombinant
arylsulfatase gene (tubulin promoter/arylsulfatase gene). After bombardment of Volvox
reproductive cells with DNA-coated gold microprojectiles, transformants
expressing arylsulfatase constitutively were recovered, indicating the presence
of the machinery for homologous recombination in Volvox. Second, a well
characterized loss-of-function mutation in the nuclear nitrate reductase gene
(nitA) with a single G --> A nucleotide exchange in a 5'-splice site was
chosen as a target for gene replacement. Gene replacement by homologous
recombination was observed with a reasonably high frequency only if the
replacement vector containing parts of the functional nitrate reductase gene
contained only a few nucleotide exchanges. The ratio of homologous to random
integration events ranged between
Raven JA
Multiple
origins of plasmodesmata
EUR J PHYCOL 32 (2): 95-101 MAY 1997
Abstract:
Plasmodesmata in photosynthetic eukaryotes are found in all embryophytes, in
many members of the Chlorophyta, and in the Phaeophyceae. The Phaeophyceae and
the Chlorophyta clearly developed cell walls and multicellularity
independently, sc that (in the absence of lateral gene transfer) plasmodesmata
evolved independently in these groups. The minimum number of independent
origins of plasmodesmata in the Chlorophyta based on molecular phylogenies is
two (Chlorophyceae sensu late, Charophyceae sensu late). Other intercellular
connections in members of the Chlorophyta (Ctenocladus, Smithsoniella, Volvox)
are structurally very different from true plasmodesmata. Recently published
taxonomies of the Chlorophyta have five classes (Chlorophyceae,
Oedegoniophyceae, Trentepohliophyceae, Klebsormidiophyceae and Charophyceae
sensu stricto) with plasmodesmata out of a total of thirteen. However, it is by
no means clear that these classes all acquired plasmodesmata independently.
Nozaki H, Ito M, Sano R, et al.
Phylogenetic
analysis of Yamagishiella and Platydorina (Volvocaceae, Chlorophyta) based on
rbcL gene sequences
J PHYCOL 33 (2): 272-278 APR 1997
Abstract:
Yamagishiella, based on Pandorina unicocca Rayburn et Starr is distinguished
from Eudorina by its isogamous sexual reproduction, whereas Platydorina
exhibits anisogamous sexual reproduction. In the present study, rue sequenced
the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL)
genes from five Japanese and North American strains of Y. unicocca (Rayburn et
Stair) Nozaki, true Platydorina caudata Kofoid strains, and two strains of
Eudorina unicocca G. M. Smith, as well as eight related colonial and
unicellular species. Phylogenetic trees were constructed based on these
sequence data and on previously published rbcL gene sequences from 23
volvocalean species in order to deduce phylogenetic relationships within the
colonial Volvocales, with particular regard to the phylogenetic positions and
status of the genera Yamagishiella and Platydorina. Two robust monophyletic
groups of the anisogamous/oogamous volvocacean species were resolved in the
maximum-parsimony tree as well as in the neighbor-joining distance tree. One of
the two groups comprises three species of Volvox section Volvox,
whereas the other is composed of other sections of Volvox as well as of
all the species of Eudorina and Pleodorina. Platydorina, however, was
positioned outside these two monophyletic groups. Therefore, derivation of the
Platydorina lineage may be earlier than that of such anisogamous/oogamous
groups, or origin of ''anisogamy with sperm, packets'' in Platydorina may De
independent of sperm packet evolution in Eudorina, Pleodorina, and Volvox.
It was also resolved with high bootstrap values that all of the Y. unicocca
strains form a monophyletic group positioned outside the large monophyletic
group including Eudorina and Pleodorina. These reject the possibility of the
reverse evolution of isogamy from anisogamy to give rise to Yamagishiella
within the lineage of Eudorina.
Liss M, Kirk DL, Beyser K, et al.
Intron
sequences provide a tool for high-resolution phylogenetic analysis of volvocine
algae
CURR GENET 31 (3): 214-227 MAR 1997
Abstract:
Three nuclear spliceosomal introns in conserved locations were amplified and
sequenced from 28 strains representing 14 species and 4 genera of volvocalean
green algae. Data derived from the three different introns yielded congruent
results in nearly all cases. In pairwise comparisons, a spectrum of
taxon-specific sequence differences ranging from complete identity to no
significant similarity was observed, with the most distantly related organisms
lacking any conserved elements apart from exon-intron boundaries and a
pyrimidine-rich stretch near the 3' splice site. A metric (SI50), providing a
measure of the degree of similarity of any pair of intron sequences, was
defined and used to calculate phylogenetic distances between organisms whose
introns displayed statistically significant similarities. The rate of sequences
divergence in the introns was great enough to provide useful information about
relationships among different geographical isolates of a single species, but in
most cases was too great to provide reliable guides to relationships above the
species level. A substitution rate of approximately 3 x 10(-8) per intron
position per year was estimated, which is about 150-fold higher than in nuclear
genes encoding rRNA and about 10-fold higher than the synonymous substitution
rate in protein-coding regions. Thus, these homologous introns not only provide
useful information about intraspecific phylogenetic relationships, but also
illustrate the concept that different parts of a gene may be subject to
extremely different intensities of selection. The intron data generated here
(1) reliably resolve for the first;time the relationships among the five most
extensively studied strains of Volvox, (2) reveal that two other Volvox
species may be more closely related than had previously been suspected, (3)
confirm prior evidence that particular isolates of Eudorina elegans and
Pleodorina illinoisensis appear to be sibling taxa, and (4) contribute to the
resolution of several hitherto unsettled issues in Chlamydomonas taxonomy.
Godl K, Hallmann A, Wenzl S, et al.
Differential
targeting of closely related ECM glycoproteins: The pherophorin family from Volvox
EMBO J 16 (1): 25-34
Abstract:
The alga Volvox carteri represents one of the simplest multicellular
organisms. Its extracellular matrix (ECM) is modified under developmental
control, e.g. under the influence of the sex-inducing pheromone that triggers
development of males and females at a concentration below 10(-16) M. A novel
ECM glycoprotein (pherophorin-S) synthesized in response to this pheromone was
identified and characterized. Although being a typical member of the
pherophorins, which are identified by a C-terminal domain with sequence
homology to the sex-inducing pheromone, pherophorin-S exhibits a completely
novel set of properties. In contrast to the other members of the family, which
are found as part of the insoluble ECM structures of the cellular zone,
pherophorin-S is targeted to the cell-free interior of the spherical organism
and remains in a soluble state. A main structural difference is the presence of
a polyhydroxyproline spacer in pherophorin-S that is linked to a saccharide
containing a phosphodiester bridge between two arabinose residues. Sequence
comparisons indicate that the self-assembling proteins that create the main
parts of the complex Volvox ECM have evolved from a common ancestral
gene.
Sumper M, Hallmann A
Biochemistry
of the extracellular matrix of Volvox
INT REV CYTOL 180: 51-85 1998