Comparison of the GAGA factor genes of Drosophila melanogaster and Drosophila virilis reveals high conservation of GAGA factor structure beyond the BTB/POZ and DNA-binding domains

Abstract

As a member of the trithorax-group, the Trithorax-like (Trl) gene of Drosophila melanogaster contributes to the expression of homeotic genes and many other genes. Trl encodes different isoforms of the GAGA factor which is thought to act as an "antirepressor" of transcription by remodelling chromatin structure and thereby rendering control regions accessible for transcriptional activators. A more global role of the GAGA factor in chromatin structure and function is suggested by various phenotypes of Trl mutations, such as modification of position effect variegation. To better define the molecular basis of these pleiotropic effects, we cloned cDNAs encoding the GAGA isoforms of D. melanogaster and a distantly related species, D. virilis. We also characterized the genomic organization of both the D. melanogaster and D. virilis genes, and analysed the expression patterns of isoform-specific mRNAs. The D. virilis GAGA isoforms show high similarity to their D. melanogaster counterparts, particularly within the BTB/POZ protein-interaction and the zinc finger DNA-binding domains. Interestingly, conservation clearly extends beyond the previously defined limits of these domains. Moreover, the comparison reveals a completely conserved block of amino acid residues located between the BTB/POZ and DNA-binding domains, and a high conservation of the C-terminus specific for one of the GAGA isoforms. Thus, sequences of as yet unknown functions are defined as rewarding targets for further mutational analyses. The high conservation of the GAGA proteins of the two species is in accord with the nearly identical genomic organization and expression patterns of the corresponding genes.

Functional Mapping of the GAGA Factor Assigns Its Transcriptional Activity to the C-terminal Glutamine-rich Domain

“…This colinearity is lost for the intermediate region. All these GAGA proteins show a strong sequence conservation at the N-terminal portion, and although they differ at the glutamine-rich C-terminal domain, they also show a similar overall structure and composition (10,30). The fact that the variable region has partially conserved the relevant motifs described above suggests that the other members of this group could also retain some transcriptional activation function.…”

GAGA Factor Isoforms Have Distinct but Overlapping Functions In Vivo

“…The GAGA-519 and GAGA-581 proteins behave indistinguishably in in vitro DNA binding assays and in tissue culture transient-transfection experiments, and they colocalize in polytene chromosomes (3). On the other hand, the striking conservation of the two isoforms between distantly related species of fruit flies (20) suggests that these isoforms may have distinct functions. This possibility is supported by the differences in the stage-and tissue-specific patterns of expression of the GAGA-519 and GAGA-581 proteins (3).…”

GAGA Factor Down-regulates Its Own Promoter

“…The study shows that GAGA is a Drosophila nuclear factor that is encoded by the Trithorax-like gene (Trl) and is of maternal effect (1). At least two isoforms, GAGA 519 and GAGA 581 , are known to be produced by alternative splicing (2), and both the sequence of the isoforms and their splicing patterns are highly conserved between Drosophila melanogaster and the distant species Drosophila virilis (3). The GAGA factors present an overall modular structure that is formed by an N-terminal BTB/POZ domain involved in protein oligomerization (4,5); a DNA binding domain, composed of a single zinc-finger and three adjacent basic regions, that confers sequence-specific DNA binding (6 -8); and a C-terminal domain (Q-domain) that in both isoforms is glutamine-rich, although the amino acid sequences differ, and is the activation domain (9 -11).…”