The Molecular Structure of the Beadex and Heldup-A Loci of Drosophila melanogaster

Year: 1987 DOI: 10.7907/c2k4-a231
Genetic studies indicate that excess-of-function Beadex mutations and loss-of-function heldup-a mutations affect different parts of a single bipartite genetic unit. In order to investigate the molecular nature of the Beadex and heldup-a mutations we isolated DNA from a 49 kilobase region surrounding the sites at which these mutations occur. We found gross structural alterations associated with one heldup-a mutation and each of the 13 different Beadex mutations that we examined. As expected from previous genetic studies these loci are separated by only a short molecular distance which is at most 1.5 kilobases. The structural alterations associated with the thirteen Beadex excess-of-function mutations examined are clustered within a three kilobase region. Several of these mutations are found to be associated with the deletion of part of an 800 bp region. This indicates that an element that normally represses gene activity is located within this region: A 200 base pair segment which is required for the function of the wild-type heldup-a locus is defined using a heldup-a mutation which results from a small deletion.

Two RNA transcripts have been found which span this 200 base pair segment. One of these two transcripts, a four kilobase RNA, is expressed during stages in which the Beadex structural gene product is expected to be active. The structure of this transcript is affected by one heldup-a mutation and each of five Beadex mutations that were examined. However, only one of the Beadex mutant alleles expresses significantly elevated levels of this RNA. Other RNA species in the region surrounding the the Beadex and heldup-a loci were not affected either in amount or size by Beadex mutations.

We have also reintroduced a wild-type 10.4 kilobase fragment, which includes the region m which Beadex and heldup-a mutations map, into the Drosophila genome by P element mediated transformation. This introduced fragment fails both to complement loss-of-function heldup-a mutations and to enhance an excess-of-function phenotype. This result indicates that some sequences required for the normal heldup-a function must be located outside this 10.4 kilobase region.