CLASS-III POU GENES - GENERATION OF HOMOPOLYMERIC AMINO-ACID REPEATS UNDER GC PRESSURE IN MAMMALS

The class III POU transcription factor genes play an important role in the nervous system. Comparison of their entire amino acid sequences d isclosed a remarkable feature of particular mammalian class III POU ge nes. Alanine, glycine, and proline repeats were present in the mammali an Brain-1 gene, whereas most of these repeats were absent in the nonm ammalian homologue. The mammalian Brain-2 gene had alanine, glycine, p roline, and glutamine repeats, which were missing in the nonmammalian homologue. The mammalian Scip gene had alanine, glycine, proline, and histidine repeats, but the nonmammalian homologue completely lacked th ese repeats. In contrast, the mammalian Brain-4 gene had no amino acid repeats like its nonmammalian homologue. The mammalian genes containi ng the characteristic amino acid repeats had another feature, higher G C content. We found a positive correlation between the GC content and the amino acid repeat ratio. Those amino acids were encoded by triplet codons with relatively high GC content. These results suggest that th e GC pressure has facilitated generation of the homopolymeric amino ac id repeats.