Transcriptional regulation of C. elegans excretory cell-expressed genes by the POU homeobox transcription factor CEH-6

Caenorhabditis elegans' excretory cell is a large cell that extends the entire length of the nematode and is functionally analogous to a kidney. In order to develop a better understanding of genes involved in the development of the excretory cell, I have characterized a transcriptional regulatory mechanism involved in driving gene expression in this cell. Seven of the 13 vertebrate Aquaporins (AQPs; water channel proteins) express in the kidney where they assist in recapturing water lost because of renal filtration. There are also 12 aqp-encoding genes in C. elegans of which three express in the excretory cell. The expression pattern of aqp-8 is limited to the excretory cell during post-embryonic developmental stages based on a GFP tagging assay. Analysis of 5' truncations of aqp-8's promoter region, coupled with interspecies comparative analyses, revealed that an octamer DNA element (ATTTGCAT) is critical for driving excretory cell expression. The octamer element associates with POU homeobox proteins of which there are three in C. elegans. The class III POU transcription factor homolog, ceh-6, is an essential gene and is the only POU member with an expression pattern that overlaps with aqp-8's. I have demonstrated, using both in vitro and in vivo approaches, that CEH-6 is the cognate transcription factor for the octamer motif. I have searched the genome for genes controlled by this transcriptional regulatory mechanism by locating interspecies conserved octamer motifs in gene-upstream regions. The candidate promoter regions were tested for their ability to drive expression in the excretory cell. I assessed the dependence on the cis-regulatory element for driving excretory cell expression by assessing the expression patterns of 5' promoter truncation constructs targeting the octamer motif. I have identified nine genes that are modulated by the CEH-6/octamer motif regulatory mechanism including the five: M176.5 (a gene of unknown function), sdr-2 (a ferric reductase), F16F9.1 (a transcription factor), twk-36 (a potassium channel), and R02F2.8 (an amino acid transporter).