MicroRNAs (miRNAs) have been found to regulate gene expression across eukaryotic species, but the function of most miRNA genes remains unknown. Here we describe how the analysis of the expression patterns of a well-conserved miRNA gene,
mir-57, at cellular resolution for every minute during early development of Caenorhabditis elegans provided key insights in understanding its function. Remarkably,
mir-57 expression shows strong positional bias but little tissue specificity, a pattern reminiscent of Hox gene function. Despite the minor defects produced by a loss of function mutation, overexpression of
mir-57 causes dramatic posterior defects, which also mimic the phenotypes of mutant alleles of a posterior Hox gene,
nob-1, an Abd homolog. More importantly,
nob-1 expression is found in the same two posterior AB sublineages as those expressing
mir-57 but with an earlier onset. Intriguingly,
nob-1 functions as an activator for
mir-57 expression; it is also a direct target of
mir-57. In agreement with this, loss of
mir-57 function partially rescues the
nob-1 allele defects, indicating a negative feedback regulatory loop between the miRNA and Hox gene to provide positional cues. Given the conservation of the miRNA and Hox gene, the regulatory mechanism might be broadly used across species. The strategy used here to explore
mir-57 function provides a path to dissect the regulatory relationship between genes.