PHYLOGENETIC DEPTH OF S10 AND SPC OPERONS - CLONING AND SEQUENCING OFA RIBOSOMAL-PROTEIN GENE-CLUSTER FROM THE EXTREMELY THERMOPHILIC BACTERIUM THERMOTOGA-MARITIMA

A segment of Thermotoga maritima DNA spanning 6,613 bp downstream from the gene tnf for elongation factor Tu was sequenced by use of a chrom osome walking strategy, The sequenced region comprised a string of 14 tightly linked open reading frames (ORFs) starting 50 bp downstream fr om tuf. The first 11 ORFs were identified as homologs of ribosomal pro tein genes rps10, rpl3, rpl4, rpl23, rpl2, rps19, rpl22, rps3, rpl16, rpl29, and rps17 (which in Escherichia coli constitute the S10 operon, in that order); the last three ORFs were homologous to genes rpl14, r pl24, and rpl5 (which in E. coli constitute the three promoter-proxima l genes of the spectinomycin operon). The 14-gene string was preceded by putative -35 and -10 promoter sequences situated 5' to gene rps10, within the 50-bp spacing between genes tnf and rps10; the same region exhibited a potential transcription termination signal for the upstrea m gene cluster (having tuf as the last gene) but displayed also the po tential for formation of a hairpin loop hindering the terminator; this suggests that transcription of rps10 and downstream genes may start f arther upstream. The similar organization of the sequenced rp genes in the deepest branching bacterial phyla (T. maritima) and among Archaea has been interpreted as indicating that the S10-spc gene arrangement existed in the (last) common ancestor. The phylogenetic depth of the T hermotoga lineage was probed by use of r proteins as marker molecules: in all except one case (S3), Proteobacteria or the gram-positive bact eria, and not the genus Thermotoga, were the deepest-branching lineage ; in only two eases, however, was the inferred branching order substan tiated by bootstrap analysis.