Transgenic embryos carrying a 3x28 bp tandem repeat in an otherwise promoter-less GFP reporter, showed expression in a pair of head neurons with dendrites extending to the tip of the head, consistent with ASJ morphology and position. However, once the embryo hatched, the number of animals with GFP-fluorescent ASJ neurons decreased dramatically. Very few L1 larvae showed expression in ASJ, but none in L2-L4 larvae or adult animals. This is most likely due to the requirement of additional cis-regulatory regions in the surrounding promoter area necessary for post-embryonic maintenance of
trx-1 expression. A bipartite motif, within the
trx-1 promoter, composed of two 6 bp elements (CAACCC and AATTAA), separated by a 3 bp linker, is absolutely required for ASJ expression. Hence, mutation of either of the two individual 6 bp elements resulted in the abolishment of GFP expression in ASJ neurons, while mutation of the 3 bp linker did not disrupt GFP expression.