Doctoral Dissertations

Date of Award

5-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Life Sciences

Major Professor

Mary Ann Handel

Committee Members

Ranjan Ganguly, John Koontz, David Brian

Abstract

Dosage compensation is a gene regulatory mechanism that equalizes the expression of X-linked genes in the two sexes despite a two-fold higher X-linked gene dosage in the females relative to males. In Drosophila, dosage compensation is accomplished through the transcriptional hyperactivation of X-linked genes in males to approximately twice that in females. The transcriptional hyperactivation is believed to be mediated through the interaction between cis-acting sequences of the X chromosome and trans-acting factors that appear to act in a male-limited manner. One of the major questions arising from this model has been the identity and properties of cis-acting dosage-compensatory sequences. So far no discrete sequence that mediates male-specific transcriptional hyperactivation has been identified. The study reported in this dissertation is the initiation of a project, the long term goal of which is to identify and characterize cis-acting sequences that mediate male-specific transcriptional hyperactivation. The focus of this study has been the X1R chromosome of D. miranda, since the homolog of this chromosome in D. melanogaster is autosomal (3L) and therefore does not face the problem of dosage effect in the two sexes. This lends an evolutionary perspective to this study, since it can be expected that the nature of modifications that a gene is subjected to due to its X-linked and dosage compensated status may be highlighted by a comparison with a homolog that is not X-linked and under no pressure to overcome a dosage difference in the two sexes. This dissertation describes the characterization of an X1R-linked gene of D. miranda with the major objective of establishing it as a system which could be subsequently exploited in the detailed characterization of cis-acting sequences that mediate dosage compensation in Drosophila. The results of this study establish that a retina-specific gene, 507, which is 3L-linked in D. melanogaster, is a potentially good system with which to address some specific questions regarding dosage compensatory cis-acting sequences: What are they? What is their physical and functional relationship with other cis-acting transcriptional control elements that influence gene transcription? In addition it is expected that a comparative study of gene 507 of D. miranda and D. melanogaster may address the question of how dosage-compensatory cis-acting sequences evolve. In this study, gene 507, was shown to be X1R-linked and dosage-compensated in D. miranda. In order to initiate the mapping of its transcriptional control regions, gene 507 of D. miranda, 507mr, was introduced into the D. melanogaster genome at autosomal sites. The transgenic 507mr is expressed in the retina-specific manner appropriate for this gene and furthermore, the steady-state levels of transgene-derived mRNA were 1.4-1.6 fold higher in males, indicating a partial response to the dosage-compensatory machinery of D. melanogaster. A nucleotide sequence comparison of the transgene with that of the D. melanogaster homolog has highlighted some interesting features which may be involved in male-specific transcriptional hyperactivation of this gene. In addition, the conceptual translational product of this gene was found to be a homolog of the vertebrate visual system protein, arrestin, which plays an important role in visual transduction.

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