Some eukaryotic genes are composed of historically separate genes that
"live together" in the form of gene fusions. For instance, Neurospora
and many other fungi have a "trp1" gene that is homologous to three
seaparate genes (trpG, trpF and trpC) of Pseudomonas putida and many
other bacteria (see Hutter, et al. 1986, Annu. Rev. Microbiol. 40, 55).
Prokaryotes also have trp gene fusions (see Crawford, 1989 Annu. Rev.
Microbiol 43, 567). For instance, E. coli has a "trpD" gene that is
homologous to trpG and trpD of P. putida. My impression is that, in
general, eukaryotes are more likely than prokaryotes to have fused
genes. The reasons for this difference between eukaryotes and
prokaryotes, and the reasons for the fusions, are not well understood.
arlin at ac.dal.ca