Early embryos of the crustacean Artemia franciscana (brine shrimp) can get into cryptobiosis under adverse environmental conditions. Under these circumstances, the embryo arrests all metabolic activities, gets dehydrated and is surrounded by a hard shell. These cysts are viable for long periods and can be activated when environmental conditions are favorable. Once the cyst is activated, the embryo resumes development and gives rise in a few hours to a swimming nauplii that continues the development, through several molts, to the adult animal. The activation of the cyst can be reproduced in the laboratory which makes of the Artemia cyst an useful model system to study the mechanisms that regulate the exit of a latent state, as well as embryonic development Activation of the cyst requires resumption of all metabolic activities, including energetic metabolism, protein synthesis, gene transcription, etc. While the mechanisms involved in the activation of the metabolism and protein synthesis have been studied with some detaii in the last years (1), little is know about the process of transcriptional activation, The only well established data is the presence of active RNApolymerases in the cyst which implies that the cysts have the enzymatic activity required for transcription (2), These data suggest that the lack of transcriptional activity in the cyst must be regulated through the accessibility of the RNA polymerases to the gene promoters. Our laboratory is trying to test this hypothesis and to get more information on the mechanisms that regulate gene transcription during the activation of the cyst In the last years we have isolated cDNAand genomic clones coding for several genes whose expression is induced during the embryonic deveiopment that immediately follows cyst activation, Tohese genes encode the SarcolEndoplasmic reticulum CaATPase (3), the ex1 subunit of the Nail<ATPase (4) and three actin isoforms (5). The mRNAs encoded by all these genes, except for on actin isoform, are expressed in the cyst but their levels are markedly increased during the first hours ot embryonic development (6). The mRNA encoded by the Actin 302 gene is not expressed in the cyst and is induced after 10 hours of development (6), These studies show that gene transcription is activated in the first hours of development, after the activation of the cyst Once the expression pattern of these genes had been characterized, we have approached the study of the mechanism invoived in the regulation of their expression through the isolation and characterization of their promoter regions. The exon~ntron structure of these genes was estabiished and their transcription initiation sites determined (3-5). The sarco/endoplasmic reticulum CaATPase gene was exceptional since this gene can be transcribe from two different promoters to give two mRNAs of different size that code for two protein isoforms differingat their C-terminal amino acids and that are expressed in different tissues (7,8).