Chloramphenicol resistance mutations in the single 23S rRNA gene of the archaeon Halobacterium halobium

Phenotypic effects of targeted mutations in the small subunit rRNA gene of Tetrahymena thermophila

Tetrahymena thermophila is an ideal organism with which to study functional aspects of the rRNAs in vivo since the somatic rRNA genes of T. thermophila can be totally replaced by cloned copies introduced via microinjection. In this study, we made small insertions into seven sites within the small subunit rRNA gene and observed their phenotypic effects on transformed cells. Two mutated genes coding for rRNA (rDNAs), both of which bear insertions in highly conserved sequences, failed to transform and are therefore believed to produce nonfunctional rRNAs. Three other altered rDNAs produce functional rRNAs that can substitute for most or all of the cellular rRNA. Two of these bear insertions in highly variable regions, and, surprisingly, the other has an insertion in a region that is well conserved for both sequence and secondary structure among eucaryotes. In addition, two other insertions appear to destabilize rRNAs that contain them. Our findings make predictions concerning the positions of some of these sites within the tertiary structure of the small ribosomal subunit and thus serve as an in vivo test of the existing tertiary structure models for the small subunit rRNA. Our results are in good agreement with expectations based on sequence comparison and in vitro work.

Introducing mutations into the single-copy chromosomal 23S rRNA gene of the archaeon Halobacterium halobium by using an rRNA operon-based transformation system

A vector-transformation system is described that permits replacement of a portion of the single rRNA operon of the archaeon Halobacterium halobium with a homologous fragment from a vector-borne gene. The vector construct contains three functional sections: (i) an entire H. halobium rRNA operon with two selective mutations in the 23S rRNA gene, the substitutions of A----G at position 1159 conferring resistance to thiostrepton and C----U at position 2471 conferring resistance to anisomycin; (ii) the complete pHSB1 plasmid from Halobacterium sp. SB3, which interferes with vector maintenance in the transformed halobacterial cells; and (iii) a segment of the pBR322 plasmid that permits vector replication in Escherichia coli. Transformation of H. halobium with the vector plasmid generates cells resistant to both anisomycin and thiostrepton that can be selected for, and discriminated from spontaneous mutants, by a two-step selection procedure. After transformation, the plasmid recombines homologously with the chromosome so that the plasmid-borne rDNA segment with resistance markers substitutes for the corresponding region of the chromosomal rRNA operon, and the transforming plasmid is lost. Eventually, this leads to a homogeneous population of the mutant ribosomes in the cell. Other mutations that are engineered in the vector-borne rRNA sequences can be transferred to the chromosomal rRNA operon concomitantly with the selective markers. The system has considerable potential for ribosomal engineering.

Functional interactions within 23S rRNA involving the peptidyltransferase center

A molecular genetic approach has been employed to investigate functional interactions within 23S rRNA. Each of the three base substitutions at guanine 2032 has been made. The 2032A mutation confers resistance to the antibiotics chloramphenicol and clindamycin, which interact with the 23S rRNA peptidyltransferase loop. All three base substitutions at position 2032 produce an erythromycin-hypersensitive phenotype. The 2032 substitutions were compared with and combined with a 12-bp deletion mutation in domain II and point mutations at positions 2057 and 2058 in the peptidyltransferase region of domain V that also confer antibiotic resistance. Both the domain II deletion and the 2057A mutation relieve the hypersensitive effect of the 2032A mutation, producing an erythromycin-resistant phenotype; in addition, the combination of the 2032A and 2057A mutations confers a higher level of chloramphenicol resistance than either mutation alone. 23S rRNAs containing mutations at position 2058 that confer clindamycin and erythromycin resistance become deleterious to cell growth when combined with the 2032A mutation and, additionally, confer hypersensitivity to erythromycin and sensitivity to clindamycin and chloramphenicol. Introduction of the domain II deletion into these double-mutation constructs gives rise to erythromycin resistance. The results are interpreted as indicating that position 2032 interacts with the peptidyltransferase loop and that there is a functional connection between domains II and V.