Unique tRNA gene profile suggests paucity of nucleotide modifications in anticodons of a deep-sea symbiotic Spiroplasma.

The position 34 of a tRNA is always modified for efficient recognition of codons and accurate integration of amino acids by the translation machinery. Here, we report genomics features of a deep-sea gut symbiotic Spiroplasma, which suggests that the organism does not require tRNA(34) anticodon modifications. In the genome, there is a novel set of tRNA genes composed of 32 species for recognition of the 20 amino acids. Among the anticodons of the tRNAs, we witnessed the presence of both U34- and C34-containing tRNAs required to decode NNR (A/G) 2:2 codons as countermeasure of probable loss of anticodon modification genes. In the tRNA fragments detected in the gut transcriptome, mismatches expected to be caused by some tRNA modifications were not shown in their alignments with the corresponding genes. However, the probable paucity of modified anticodons did not fundamentally change the codon usage pattern of the Spiroplasma. The tRNA gene profile that probably resulted from the paucity of tRNA(34) modifications was not observed in other symbionts and deep-sea bacteria, indicating that this phenomenon was an evolutionary dead-end. This study provides insights on co-evolution of translation machine and tRNA genes and steric constraints of codon-anticodon interactions in deep-sea extreme environment.