bicoid mRNA localization signal: phylogenetic conservation of function and RNA secondary structure

Transcripts of the bicoid (bcd) gene are localized to the anterior pole of the Drosophila oocyte, thereby allowing formation in the embryo of an anteroposterior gradient of the bcd protein morphogen. We previously showed that a 630 nucleotide portion of the 3' noncoding region of the bcd mRNA is necessary for this localization, and is sufficient to confer anterior localization on a heterologous transcript. Here I have used a comparative analysis to begin to more precisely define the cis-acting mRNA localization signal. The bcd genes from six additional Drosophila species were cloned, and DNA of the 3' noncoding regions sequenced. Three of these regions were tested interspecifically for mRNA localization in D. melanogaster and each functioned correctly; these regions must therefore contain the cis-acting signal. The primary sequences, which are up to 50% divergent from the D. melanogaster gene, show patchy homology throughout most of the region. Interestingly, all seven species can potentially form a large stereotypic secondary structure. This structure is a likely candidate for the localization signal and can be used for the rational design of mutations to test that possibility.