The direct ancestor of the DNA-protein world of today is considered to have been an RNA-peptide world, in which peptides were co-factors of RNA-mediated catalysis and replication. Evidence for these ancestral peptides, from which folded proteins evolved, can be derived even today from regions of local sequence similarity within globally dissimilar folds. One of these is the 45-residue motif common to both folds of the hnRNP K homology (KH) domain.


In a survey of KH domains, we found a third fold that contains the KH motif at its core. This corresponds to the Small Domain of bacterial Ribonucleases G/E and, like type I and type II KH domains, it cannot be related to the others by a single genetic event, providing further support for the KH motif as an ancestral peptide predating folded proteins.

Supplementary information

Supplementary data are available at Bioinformatics online.

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