Highly divergent 16S rRNA sequences in ribosomal operons of Scytonema hyalinum (Cyanobacteria)

A highly divergent 16S rRNA gene was found in one of the five ribosomal operons present in a species complex currently circumscribed as Scytonema hyalinum (Nostocales, Cyanobacteria) using clone libraries. If 16S rRNA sequence macroheterogeneity among ribosomal operons due to insertions, deletions or truncation is excluded, the sequence heterogeneity observed in S. hyalinum was the highest observed in any prokaryotic species thus far (7.3–9.0%). The secondary structure of the 16S rRNA molecules encoded by the two divergent operons was nearly identical, indicating possible functionality. The 23S rRNA gene was examined for a few strains in this complex, and it was also found to be highly divergent from the gene in Type 2 operons (8.7%), and likewise had nearly identical secondary structure between the Type 1 and Type 2 operons. Furthermore, the 16S-23S ITS showed marked differences consistent between operons among numerous strains. Both operons have promoter sequences that satisfy consensus requirements for functional prokaryotic transcription initiation. Horizontal gene transfer from another unknown heterocytous cyanobacterium is considered the most likely explanation for the origin of this molecule, but does not explain the ultimate origin of this sequence, which is very divergent from all 16S rRNA sequences found thus far in cyanobacteria. The divergent sequence is highly conserved among numerous strains of S. hyalinum, suggesting adaptive advantage and selective constraint of the divergent sequence.本研究通过克隆文库（clone library）技术，在目前界定为透明席藻（Scytonema hyalinum，念珠藻目（Nostocales），蓝细菌门（Cyanobacteria））的物种复合群的5个核糖体操纵子（ribosomal operon）之一中，发现了一条高度分化的16S核糖体RNA基因（16S rRNA gene）。若排除因插入、缺失或截短导致的核糖体操纵子间16S rRNA序列宏观异质性，透明席藻中观测到的序列异质性达7.3%~9.0%，为目前已报道的所有原核生物中最高水平。这两个分化型操纵子所编码的16S rRNA分子的二级结构几乎完全一致，提示其可能具备生物学功能。本研究还对该复合群中的部分菌株开展了23S核糖体RNA基因（23S rRNA gene）分析，结果显示其与2型操纵子中的对应基因分化程度极高（序列差异率为8.7%）；且1型与2型操纵子所编码的23S rRNA分子的二级结构同样近乎一致。此外，在大量菌株中，16S-23S内部转录间隔区（16S-23S ITS）在不同操纵子间均呈现出稳定的显著差异。两类操纵子均携带符合原核生物功能性转录起始共识序列要求的启动子序列。目前认为，该基因序列起源最可能的解释是来自另一种未知异形胞蓝细菌的水平基因转移（horizontal gene transfer），但这无法解释该序列的最终起源——其与目前已在蓝细菌中发现的所有16S rRNA序列的分化程度均极高。该分化型序列在透明席藻的大量菌株中高度保守，提示该序列具有选择优势并受到选择约束。