PATTERNS OF BASE COMPOSITION WITHIN THE GENES OF DROSOPHILA-MELANOGASTER

Base composition is not uniform across the genome of Drosophila melano gaster. Earlier analyses have suggested that then is variation in comp osition in D. melanogaster on both a large scale and a much smaller, w ithin-gene, scale. Here we present analyses on 117 genes which have re liable intron/exon boundaries and no known alternative splicing. We de tect significant heterogeneity in G+C content among intron segments fr om the same gene, as well as a significant positive correlation betwee n the intron and the third codon position G+C content within genes. Bo th of these observations appear to be due, in part, to an overall decl ine in intron and third codon position G+C content along Drosophila ge nes with introns. However, there is also evidence of an increase in th ird codon position G+C content at the start of genes; this is particul arly evident in genes without introns. This is consistent with selecti on acting against preferred codons at the start of genes.