Halobacterium halobium mutations

The number of rRNA operons in archaea is always small. Haloferax vol-canii, Methanobacterium thermoautotrophicum and Methanothermus fervidus have two, and Methanobacterium vannielii has four (reviewed by Brown et al. [5]). On the basis of Southern transfers of pulsed field gels, Sanz et al. [106] report that the halophilic archaea have from one to four rRNA operons Haloarcula californiae, 4 Haloferax gibbonsii, 4 Halobacterium halobium NCMB 111, 3 Halobacterium marismortui, 3 Halococcus morrhuae, 2 and Halobacterium salinarium, 1. Bacteria have from one to eleven copies (e.g.. Bacillus subtilis, 11 [107] Escherichia coli, 7 [108], and Mycoplasma pneumoniae, 1 [109]). In Halobacterium halobium, the presence of only one rRNA operon facilitated the isolation of strains with mutated 23S rRNA genes, which are resistant to thiostrepton [110], anisomycin [111] or chloramphenicol [112]. 

Independent support for the validity of the foregoing component analysis is provided by experiments carried out with a mutant bacteriorhodopsin. Purple membranes were isolated from a mutant strain of Halobacterium halobium in which a point mutation at residue 212 (aspartic acid replaced by asparagine) was carried out by a new method of site-directed mutagenesis and expression (43, 44). The photosignal was found to be pH-independent in the range of pH 4-11 (45, 46). This photosignal was found to be a pure B1 component because its time course could be superimposed, after normalization, with that of the pure B1 component observed in a multilayered mutant bacteriorhodopsin film. Thus, Nature does indeed decompose the photosignal in accordance with the outlined component analysis. In other words, the B1 component as defined is indeed a natural entity. 

Fig. 2. The peptidyltransferase center. The structure of the central loop of Domain V of E. coli 23S rRNA is shown. Nucleotides involved in resistance against different inhibitors are indicated. Closed symbols indicate resistance and open symbols protection against chemical modification by bound antibiotic. Mutations that confer resistance to anisomycin in archaea are indicated [87] (Hcu, Halobacterium cutirubrum Hha, H. halobium). The presence of either a G or U at position 2058 in archaea is also indicated. As a consequence of this change archaea are resistant to erythromycin (Hmo, Halococcus morrhuae, Mva, Methanococcus vannielii Tte, Thermoproteus lenax Dmo, Desulfurococcus wofirfo) [29,30,88,90]. Positions where crosslinking to photoreactive derivatives of Phe-tRNA and puromycin have been observed as well as nucleotides protected by bound tRNA are also indicated. Modified from ref [73].

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