The distribution of (A,T)-rich regions has been investigated on a 835kb DNA fragment (D835), from the Drosophila X sex chromosome, unsequenced but provided with detailed physical maps for restriction enzymes-recognizing (A,T)- or (C,G)- or (mixte)-motifs, and on several sequenced DNA fragments, 100-600 kb long, from the autosomal chromosomes. Numerous (A,T)-rich regions are present in all DNA fragments. Their size varies from 0.2 kb to several kb in all cases, except for D835 where some of them extend to 20-30 kb. The relationship between these (A,T)-rich regions and several chromosome landmarks has been examined in the particular case of D835. Topo II in-vitro sites are randomly distributed with regard to (A,T)-richness. However, transcription units and repeated regions are significantly localized outside (A,T)-rich regions. On the opposite, SARs and ARSs are mostly localized within (A,T)-rich regions. Lastly, topo II in-vivo sites are almost exclusively localized in (A,T)-rich regions. Speculations are proposed on why and how (A,T)-rich regions may have appeared during the emergence of Drosophila genome from a primitive genome.
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