Table 2. Global distribution of genes for known Ψ synthases, the TruB/Cbf5p-related protein PsuX, and box H/ACA snoRNP proteins within the three domains of life.

Gene family	Bacteria	Archaea	Eucarya
RluA, RsuA	yes (for rRNA or rRNA/tRNA)	no apparent homologs	yes (RluA only; for organellar rRNA?a)
Cbf5p	no apparent homologs	yes	yes (for cytoplasmic rRNA)
Other box H/ACA snoRNP proteins (Gar1p, Nhp2p, Nop10p)	no apparent homologsb	yesc	yes
TruB	yesd (for tRNA)	no apparent homologs	yes (Pus4p) (S.cerevisiae for tRNA, mammals for ?)e
PsuX	no apparent homologs	yesf	no (S.cerevisiae) yes (C.elegans, D.melanogaster, mammals, plants, G.intestinalis)
TruA	yes (for tRNA)	yes	yes (for tRNA or tRNA/snRNAg)

^aThought to act on mitochondrial rRNA S.cerevisiae (79,93).

^bSome eubacteria do have genes for homologs of eukaryotic Nhp2p (101,102).

^cFor summary of gene organization, see text and Figure 7.

^dHomologous gene is missing in some eubacteria (see text).

^ePsuY, a distant relative of TruB, is also encoded by the H.sapiens, D.melanogaster and C.elegans genomes (see text).

^fFor description of gene organization, see text and Figure 7.

^gRef. 106.