Functions of horizontally transferred genes

Horizontal gene transfer (HGT) is thought to play an important role in the evolution of species and innovation of genomes. Different researchers examined functional propensity in HGT of protein families.

First, Jain et al. (1999) proposed under the complexity hypothesis that HGT may have occurred preferentially among operational genes (those that maintain cell growth such as metabolism-related genes) than among informational genes (those involved in DNA replication, transcription, and translation) which are part of more complex protein-interaction networks.

Second, Nakamura et al. (2004) observed that only parts of genes in functional categories such as mobile element, cell surface, DNA binding, and pathogenicity-related, were preferred.

Third, Beiko et al. (2005) found extensive evidence for the preferential transfer of metabolic genes, while informational genes (e.g. ribosomal proteins, and proteins involved in DNA replication and repair, cell wall synthesis, and cell division) are susceptible or resistant to HGT.

Recently, Choi et al. (2007) suggested that there is no strong preference of HGT for protein families of particular cellular or molecular functions. They reconfirmed previous findings that HGT was biased toward cell surface and DNA binding functions (Nakamura et al., 2004), but the biases are marginal. They suggest that HGT is nearly neutral to all genes and that a random HGT process is followed by selection due to environment or other factors.

These discrepancies may be due to differences in the methods (e.g. phylogenetic versus compositional methods) and databases used, the genome samples tested, and possibly other reasons. For example, functions were assigned to protein families by using different databases: that is, (i) The Institute for Genomic Research role categories database (Peterson et al., 2001), (ii) The NCBI clusters of orthologous groups (COG) database (http://www.ncbi.nlm.nih.gov/COG/), and (iii) Gene Ontology (GO) terms (Camon et al., 2003).

This has inspired us to examine functional correlates of the vertically versus horizontally transferred genes using uniform approaches (methods and databases).

REFERENCES:
Jain R, Rivera MC, Lake JA. Proc Natl Acad Sci U S A. 1999 Mar 30;96(7):3801-6. Horizontal gene transfer among genomes: the complexity hypothesis.

Nakamura Y, Itoh T, Matsuda H, Gojobori T. Nat Genet. 2004 Jul;36(7):760-6. Biased biological functions of horizontally transferred genes in prokaryotic genomes.

Beiko RG, Harlow TJ, Ragan MA. Proc Natl Acad Sci U S A. 2005 Oct 4;102(40):14332-7. Highways of gene sharing in prokaryotes.

Choi IG, Kim SH. Proc Natl Acad Sci U S A. 2007 Mar 13;104(11):4489-94. Global extent of horizontal gene transfer.

Dr. Haruo Suzuki
University of Idaho