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Project Progress

Bacterial homologs for alpha- and beta-tubulin genes have been found in Prosthecobacter species that are members of the Division Verrucomicrobia. The genes that we have named, btuba and btubb, occur in an operon along with a light chain kinesin homolog. radiative transfer polymerase chain reaction (RT-PCR) indicates the genes are being expressed. Phylogenetically, the deduced amino acid sequences indicate these proteins are 30 to 35% related to eukaryotic tubulins. The results of this work were published in PNAS (Jenkins et al., 2002). We are now working on resolving the various possibilities that could explain the origin of these genes. Two hypotheses involve horizontal gene transfer (HGT) either from the Eucarya to the Verrucomicrobia or vice versa. An alternative hypothesis is that a member of the Verrucomicrobia or a closely related group evolved to become the founding ancestors of the Eucarya. Based upon partial genome sequences that are available, we are currently trying to determine which of the three hypotheses seems most reasonable. The most readily tested hypothesis is the last one. If it is correct, then we would expect that a substantial number of eukaryotic genes would be found in the genomes of the Verrucomicrobia. Preliminary results based upon BLAST searches of genes of the Verrucomicrobia indicate, however, that fewer other eukaryotic genes have been found in the Prosthecobacter genome than expected based on the simplest variant of this model. However, it may be possible to consider the last hypothesis in the framework of the Pre-Darwinian world model developed by Carl Woese. His model of early cellular evolution involves a Pre-Darwinian period, dominated by rampant HGT, which preceded the Darwinian world of vertical or hereditary evolution. We are analyzing this scenario to determine whether it might help explain the origin of the Eucarya from this early period.

Reference:

Jenkins, C., R. Samudrala, I. Anderson, B. P. Hedlund, G. Petroni, N. Michailova, N. Pinel, R. Overbeek,, G. Rosati and J. T. Staley. 2002. Genes for the cytoskeletal protein tubulin in the bacterial genus Prosthecobacter. PNAS 99:17049-17054.