THE NUCLEAR GENOMES OF AFRICAN AND AMERICAN TRYPANOSOMES ARE STRIKINGLY DIFFERENT

We have investigated the compositional distributions of exons and thei r different codon positions, as well as the codon usage and amino-acid (aa) composition of the nuclear genomes of the African and American t rypanosomes Trypanosoma brucei and T. cruzi. Very large differences be tween the two species were found in all the properties investigated. T he most striking differences concern the compositional distributions o f third codon positions and the extremely large nucleotide divergence of third codon position for homologous genes encoding proteins that ar e highly conserved in their aa sequences. Moreover, if coding sequence s from each species are divided into two groups according to the GC le vels in third codon positions, very different codon usages and aa comp ositions are found. This indicates a compositional compartmentalizatio n in both genomes which had previously been detected in T. brucei (and T. equiperdum) by compositional fractionation.