Simultaneous Horizontal Gene Transfer of a Gene Coding for Ribosomal Protein L27 and Operational Genes in Arthrobacter Sp. Santiago Garcia-VallveÂ, Francesc X. SimoÂ, Miguel A. Montero, LluõÂs Arola, Antoni Romeu Evolutionary Genomics Group, Department of Biochemistry and Biotechnology, Rovira i Virgili University, Pl. Imperial TaÁrraco 1, E-43005 Tarragona, Catalonia, Spain Received: 11 February 2002 / Accepted: 6 June 2002 Abstract. Phylogenetic analysis of bacterial L27 ribosomal proteins showed that, against taxonomy, the L27 protein from the Actinobacteria Arthrobacter sp. clusters with protein sequences from the Bacillus group. The L27 gene clusters in the Arthrobacter sp. genome with six genes responsible for creatinine and sarcosine degradation. Phylogenetic analyses of or- thologue proteins encoded by three of these genes also showed a phylogenetic relationship with Bacillus species. Comparisons between the synonymous codon usage of the Arthrobacter sp. genes and those from complete genomes showed that Arthrobacter genes encoding the L27 ribosomal protein and the proteins responsible for the degradation of creatinine and sarcosine have a codon usage that is more similar to that of Bacillus species than that of Arthrobacter. We suggest that the Arthrobacter sp. genes encoding the L27 ribosomal protein and the proteins responsible for the degradation of creatinine and sarcosine were acquired simultaneously through horizontal gene transfer from an unknown Bacillus species. Key words: Horizontal gene transfer Ð Codon bias Ð L27 ribosomal protein Introduction 1 Horizontal gene transfer HGT) is the physical ex- change of genetic material between genomes. It has long been recognized as a promoter of prokaryotic genome evolution Doolittle 1999; Koonin et al. 2001; Brochier et al. 2002). As most prokaryotic genes are regulated in operons, a given set of clus- tered genes on the chromosome would probably be mobilized in block if they were the object of an HGT event. The selective advantage would guide the fate of the transferred genes once they had been incorpo- rated into the receiving genome Ochman et al. 2000). The complexity hypothesis of HGT among genomes states that extensive horizontal transfer occurs for genes involved in housekeeping operationals), whereas those involved in transcription, translation, and related processes informationals) are seldom horizontally transferred Jain et al. 1999). One group of informational genes is those that encode the ribosomal proteins r-proteins). Bacterial ribosomes contain about 35% protein, and the variety of r-proteins is enormous. In bacteria, an increased cellular demand for protein synthesis is met by in- creasing the number of ribosomes rather than by al- tering the activity of individual ribosomes Green and Noller 1997). The proportion of cellular resources devoted to making ribosomes is so large, and the function of ribosomes so important, that cells must coordinate the synthesis of r-proteins and rRNA Green and Noller 1997). Genes that encode the r- proteins occur in operons, which also contain the genes for the subunits of DNA primase, RNA poly- merase, and protein elongation factors Nomura 1984; Moore 1998). Individually, most of these r-proteins are small and highly conserved. Given the Correspondence to: A. Romeu; email: romeu@quimica.urv.es J Mol Evol 2002) 55:632±637 DOI: 10.1007/s00239-002-2358-5