Ultrabithorax protein expression in breakpoint mutants: localization of single, co-operative and redundant cis...

The correct function of homeotic genes depends on their correct spatial deployment which is regulated at the level of transcription by a number of cis regulatory elements. The localisation of cis regulatory regions has been difficult because of the enormous length of the homeotic genes. Here we try to find new cis regulatory elements by analysing Ultrabithorax expression in a number of breakpoint mutations spanning the whole Ultrabithorax gene. Most mutations breaking in the transcription unit give rise to RNAs that translate into mutant protein products. These can be detected with an antibody directed against the part of the protein encoded by the 5' exon of the gene. Using this antibody we have studied how breakpoints both upstream and downstream of the Ubx promoter affect the distribution of UBX protein. We define localised regions of DNA which are required to drive spatial expression in parasegment 5 and in the posterior compartment of parasegment 6. In contrast, our results suggest that the Ubx regulatory elements for parasegments 7-12 are highly redundant since (1) double mutants affecting both the up-stream and down-stream regulatory regions have a synergistic effect in these parasegments, and (2) in some cases, lack of Ubx expression in early development caused by the absence of certain elements is overcome by the later activity of the remaining cis elements. Our analysis of mutations breaking upstream of the transcription unit reveals that ectodermal and mesodermal Ubx expression in the posterior abdomen is regulated by co-operative cis regulatory elements. We also show that, at least, some of these cis regulatory elements are germ layer specific and not parasegment specific.