Strongylocentrotus
purpuratus Genes in Development: Homeobox Genes
SpHmx
Function
SpHmx encodes a transcription factor of H6 subfamily of homeobox proteins.
SpHmx functions as a cell-type-specific regulator
of pigment cells differentiation.
This gene is present in a single copy per haploid genome (Martinez et al., 1997).
Protein
SpHmx is a homeodomain protein. Adjacent to homeodomain
in the SpHmx amino acid sequence is a putative bipartite
nuclear localization site.
SpHmx homeodomain belongs to H6 subfamily of homeodomains (Martinez, 1997).
GenBank: 1332413
Subcellular location
Cytoplasm and perinuclear regions of pigment cells (Martinez, 1997).
Expression Pattern
SpHmx is transcribed during oogenesis and is represented
as a maternal transcript, the prevalence of which is about
4,000 copies per egg. The level of transcript then
declines abruptly, and it is undetectable from 4 hr
postfertilization until 12 hr.
During the next 12 hr there is a steady accumulation of SpHmx
transcripts. At 24 hr (blastula) the number of transcripts
per embryo is approximately 12,000. After 24 hr the levels decrease
slowly until 72 hr, the last point analysed, when
4000 transcripts per embryo were detected (Martinez et al., 1997).
At the 36-hr midgastrula stage transcripts are
detected by whole mount in situ hybridization throughout
the archenteron, including the blastopore rim. No staining is observed in sceletogenic
mesenchyme, or in ectoderm, though some staining is detected in
the mesenchymal cells just beneath the apical region of
the oral ectoderm. The highest level of mRNA at this
stage is in the delaminating secondary mesenchyme. However close inspection reveals
that only some, and not all secondary mesenchyme cells
contain SpHmx transcripts.
In 48-late gastrula-stage embryos transcripts are
confined strictly to pigment cells.
This pattern is stable and persists at least into the
pluteus stage.
In 48 hr embryo there are about 360 SpHmx transcripts per average
pigment cell. This value falls to about 140 transcripts per average pigment
cell by the end of embryogenesis.
SpHmx continues to be transcribed in the pluteus-stage
embryo and shows clearly a pigment cell-specific pattern of expression
(Martinez et al., 1997).
mRNA level
Temporal accumulation (transcript prevalence)
Method: single-stranded probe excess titration procedure
(as described
by Lee and Costlow, 1987)
Reference: Martinez et al. 1997
Stage
Egg
4-12 hr
24 hr
72 hr
Level (transcripts/embryo)
400 copies
undetectable
12000 copies
4000 copies
Spatial localization
Method: whole mount in situ hybridization
Reference: Martinez et al., 1997
Stage
36 hr blastula
48 hr gastrula
72 hr pluteus
Tissue
archenteron, mesenchymal cells beneath the apical region of
OE, some cells of secondary mesenchyme
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