Abstract
The metallothionein-B genes of the sea urchin Strongylocentrotus purpuratus encode a metallothionein (MT) isoform distinguishable from the MTA isoform. The MTB subfamily consists of at least two genes, MTB1 and MTB2, and possibly two to three others. The unique MTB1 and MTA genes have a high degree of identity but diverge in structural detail and expression. Transcripts of the MTA, MTB1, troponin C Spec 1, and CyIIIa actin genes begin simultaneously to accumulate at an early blastula stage. MTB1 mRNA becomes localized in the embryonic gut and oral ectoderm, whereas MTA, Spec 1, and CyIIIa actin mRNAs are spatially restricted to the aboral ectoderm. Several DNA elements are localized at the same positions in the MTB1 and MTA genes: these include respective CATA and TATA boxes, two metal response elements, and three distinct upstream DNA elements that are also present, and in the same order, in the Spec 1 gene promoter. A heptameric sequence, element A, is present at two sites each in the Spec 1 and CyIIIa actin genes, five sites in MTA, but only one site in MTB1. Most strikingly, the first intron of MTA contains elements not found in the MTB1 introns, including a consensus metal response element, an element A, and the P3A site demonstrated in the CyIIIa actin gene to be linked to the regulation of spatial expression.
Highlights
The metallothionein-Bgenes of the sea urchin Strongylocentrotuspurpuratus encode a metallothionein (MT) isoform distinguishable from the MTA isoform
Incertaincasestherestricted expression has been attributed to DNA methylation [9, 11, 14], offering a potential mechanism for developmental switching[9].Whereas M T genes areprimarilyactivated through highly conserved metal response elements (MREs), their activitiesmay be linked to other gene activities through common DNA elements, such as glucocorticoid response elements and G boxes [15], and a variety of sites for transcription factors[16], such as Spl, AP-1o,r NF1 [8, 17].Suchlinkages of M T genesmightalso involve sensus metal response element, an element A, and the various histospecific sets of genes
A multiplicity of start sites is not unusual amongM T genes (similar patterns having been observed for SpMTA [5] and for human M T genes [9] the heterogenity in these assays could be due to either RNA degradation or steric hindrance by the mRNA cap structure
Summary
The costs of publication of this several genes t h a t areexpressed in this tissuew, hich includes article were defrayed in part by the payment of page charges. This Spec 1 [20], CyIIIa actin [21], Spec 2 [22], and arylsulfatase article must be hereby marked “advertisement” in accord- [23, 24]. We have isolated a genedesignated MTB, that diffefrosm a previously described MTB cDNA [18]but encodes the same MTB isoform Wehave used this gene to develop a unique MTB1 probe to monitor the spateixapl ression of its encoded mRNA unambiguously. 30606), inordertoestablish a basis for evaluatingtheir contrasting patternsof expression
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