Hyperthermus butylicus, a hyperthermophilic neutrophile and anaerobe, is a 
member of the archaeal kingdom Crenarchaeota. Its genome consists of a single 
circular chromosome of 1,667,163 bp with a 53.7% G+C content. A total of 1672 
genes were annotated, of which 1602 are protein-coding, and up to a third are 
specific to H. butylicus. In contrast to some other crenarchaeal genomes, a high 
level of GUG and UUG start codons are predicted. Two cdc6 genes are present, but 
neither could be linked unambiguously to an origin of replication. Many of the 
predicted metabolic gene products are associated with the fermentation of 
peptide mixtures including several peptidases with diverse specificities, and 
there are many encoded transporters. Most of the sulfur-reducing enzymes, 
hydrogenases and electron-transfer proteins were identified which are associated 
with energy production by reducing sulfur to H(2)S. Two large clusters of 
regularly interspaced repeats (CRISPRs) are present, one of which is associated 
with a crenarchaeal-type cas gene superoperon; none of the spacer sequences 
yielded good sequence matches with known archaeal chromosomal elements. The 
genome carries no detectable transposable or integrated elements, no inteins, 
and introns are exclusive to tRNA genes. This suggests that the genome structure 
is quite stable, possibly reflecting a constant, and relatively uncompetitive, 
natural environment.