GART
The Gart locus encodes a multienzyme protein in which a single polypeptide performs multiple enzymatic functions (Davidson & Peterson 1997). The three enzymes are physically linked (but separated by introns) and transcribed as a single large polypeptide molecule. The Gart locus encodes the enzymes involved in the second, third, and fifth steps in de-novo synthesis of purines: glycinamide ribotide synthetase (GARS), aminoimadizole ribotide synthetase (AIRS), and glycinamide ribotide transformylase (GART). This organization (GARS-AIRS-GART) is apparently conserved among invertebrates as well as vertebrates but the three genes are physically separated in yeasts and bacteria (Davidson & Peterson 1997).
The Gart locus is located on the second chromosome in Drosophila melanogaster (Moriyama & Gojobori 1989) and consists of several exons interrupted by introns of variable lengths. In two species of Drosophila analyzed (D. melanogaster and D. pseudobscura) there is a functional gene located within the largest of the GART introns (the pupal cuticle protein gene [PCP] -- Henikoff et al. 1986a,b; Henikoff & Eghtedarzadeh 1987). This "nested" gene is fully functional and transcribed in the opposite direction from the Gart gene. This bizarre configuration is apparently unique to Drosophila and relatives because it is lacking in Chironomus (Clark & Henikoff 1992). Furthermore, comparison of the Chironomus and Drosophila Gart loci indicated that there are differences in the location of introns suggesting that intron insertion/deletions have occurred. Clark & Henikoff (1992) provide a useful map of the introns and exons within this locus in both Chironomus and Drosophila. The coding region of the Gart locus is over 4000 bp in length and could be a useful gene for higher-level insect phylogeny (Moulton 2003).
The available fly sequences are Drosophila melanogaster (Genbank accession number NM078773) and Chironomus tentans (Genbank accession number S43653). We have not been able to locate the Gart locus in our blast searches of the honey bee genome, however, this should be a promising gene for bee phylogeny because is has a relatively high rate of non-synonymous substitutions (like CAD) and because it could potentially yield an extremely large data set (>4000 bp).
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