Expression and Function of Different Guanine-Plus-Cytosine Content 16S rRNA Genes in at Different Temperatures.

The halophilic archaeon harbors three ribosomal RNA (rRNA) operons (, , and ) that contain the 16S rRNA genes , , and , respectively. Although and () have almost identical sequences, the and sequences differ by 5.4%, and they differ by 2.5% with respect to guanine-plus-cytosine content (). The strong correlation between the typical growth temperatures of archaea and of their 16S rRNA genes suggests that may harbor different 16S rRNA genes having different to maintain rapid growth in a wide range of temperatures. We therefore performed reverse transcription-coupled quantitative PCR to assess expression levels of (, 58.9%) and (, 56.4-56.5%) at various temperatures. The expression ratio of to increased with culture temperature. Mutants with complete deletions of one or two of the three rRNA operons were constructed and their growth rates at different temperatures compared to that of the wild-type. The growth characteristics of the rRNA operon single-mutant strains were indistinguishable from the wild-type. The rRNA operon double-mutant strains maintained the same temperature range as wild-type but displayed reduced growth rates. In particular, the double-mutant strains grew much slower than wild-type at low temperature related to minimum growth temperature of the wild-type. On the other hand, at physiologically high temperatures the wild-type and the double-mutant strain which harbors only with high- grew significantly faster than the double-mutant strain which harbors only with low-. These findings suggest the importance of 16S rRNAs transcribed from with high- in maintaining rapid growth of this halophilic archaeon at raised growth temperatures.