Coping with cadmium exposure in various ways: the two helicid snails Helix pomatia and Cantareus aspersus share the metal transcription factor-2, but differ in promoter organization and transcription of their Cd-metallothionein genes.
Gastropods are able to withstand fluctuating availabilities of nonessential trace elements such as cadmium by induction of Cd-specific metallothionein isoform (Cd-MT) expression. As in other species, the induction mechanism involves the binding of metal-regulatory transcription factors (MTF-1 or MTF-2) to metal responsive elements (MREs) in the MT promoter regions. Cd-dependent transcription of Cd-MT genes was assessed by quantitative real time PCR in two helicid gastropods, Helix pomatia and Cantareus aspersus, over a period of eight days. The promoter regions of the Cd-MT genes of the two species were sequenced and compared regarding the position of MREs and other relevant potential transcription factor binding sites (TFBs). Cd-MT gene transcription is induced after Cd exposure in Helix pomatia and Cantareus aspersus, showing a transient peak in Helix pomatia, contrasting with a persistent induction rate in Cantareus aspersus. Since the existence of MTF-2 was verified in both species, differing transcription patterns of Cd-MT genes must be due to functional differences in their metal-responsive promoter regions. Both promoters contain a proximal cluster of three MREs overlapping with TFBs for the transcriptional regulator Sp1. In contrast to Cantareus aspersus, however, the Cd-MT gene of Helix pomatia hosts an additional distal MRE overlapping with a Sp1 binding site and a CACCC box. Inhibitory effects of MRE overlapping Sp1 binding sites were observed in other MT genes. We therefore suggest that transient Cd-MT transcription upon Cd(2+) exposure in Helix pomatia may be the result of an inhibitory action of the distal MRE cluster.
Höckner
M
M
Institute of Zoology, University of Innsbruck, Innsbruck, Austria.
Stefanon
K
K
Schuler
D
D
Fantur
R
R
de Vaufleury
A
A
Dallinger
R
R
D000222:Adaptation, Physiological; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002104:Cadmium / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D008668:Metallothionein / Q000235:genetics / Q000378:metabolism*; D019216:Metals, Heavy / Q000378:metabolism* / Q000494:pharmacology; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic / Q000235:genetics*; D017385:Sequence Homology; D012908:Snails / Q000235:genetics / Q000378:metabolism*; D013045:Species Specificity; D018709:Statistics, Nonparametric; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology
D019216:Metals, Heavy; D014157:Transcription Factors; D002104:Cadmium; D008668:Metallothionein