Extremely divergent histone H4 sequence from trypanosoma cruzi: Evolutionary implications

Trypanosoma cruzi presents six histones electrophoretically resolved in three gel systems. Indirect evidence shows that one of these histones, name, corresponds to H₄ in other species. We present evidence that histones is H₄ by sequencing its amino terminal end. The amino terminal of T. cruzi histone H₄, unlike that of other H₄s examined thus far is not blocked. Moreover, this protein presents two variants. This partial amino acid sequence of T. cruzi histone H₄ differs greatly from homologous sequences of human, yeast, or Tetrahymena. Since the conservatism of the core histones (H₂A, H₂B, H₃, and H₄) is clearly illustrated by comparative sequence analyses, the data shown here demonstrates that T. cruzi histone H₄ is the most divergent reported. Quantitative analysis of the data suggests that the rate of substitutions in the histone H₄ amino terminal sequence varies among different lineages. We postulate a slow-down in the evolutionary rate of histone H₄ amino terminal domain in the metazoa branch related perhaps to the appearance of a novel function for this domain.