This family consists of bacterial proteins encoded within an intervening sequence present within some 23S rRNA genes [1-3]. It folds into an anti-parallel four-helix bundle and forms homopentamers [4].

1: Intervening sequence with conserved open reading frame in eubacterial 23S rRNA genes. Ralph D, McClelland M; Proc Natl Acad Sci U S A 1993;90:6864-6868. PMID:8341711

2: Characterization of the 23S and 5S rRNA genes of Coxiella burnetii and identification of an intervening sequence within the 23S rRNA gene. Afseth G, Mo YY, Mallavia LP; J Bacteriol 1995;177:2946-2949. PMID:7751314

3: The unusual 23S rRNA gene of Coxiella burnetii: two self-splicing group I introns flank a 34-base-pair exon, and one element lacks the canonical omegaG. Raghavan R, Miller SR, Hicks LD, Minnick MF; J Bacteriol. 2007;189:6572-6579. PMID:17644584

4: Crystal structure of the conserved hypothetical cytosolic protein Xcc0516 from Xanthomonas campestris reveals a novel quaternary structure assembled by five four-helix bundles. Lin LY, Ching CL, Chin KH, Chou SH, Chan NL; Proteins. 2006;65:783-786. PMID:16948161