Six ribosomal protein genes from the sulfur dependent extreme thermophilic archaebacterium Sulfolobus solfataricus were cloned and sequenced. Four of these genes code for proteins that are equivalent to ribosomal proteins L11, L1, L10 and L12 in Escherichia coli. The other two genes code for proteins that have no equivalent in the eubacteria. The product of one of these genes was found to be equivalent to ribosomal proteins L46 from yeast (Leer et al. 1985a) and L39 from rat liver (Lin et al. 1984), while the product of the other gene shows no sequence similarity to any of the ribosomal proteins present in the data base. In Sulfolobus, the genes that code for ribosomal proteins L11, L1, L10 and L12 are organized in the same order as in Escherichia coli, that is 5' L11, L1, L10, L12 3'. The major transcript from this gene cluster was found to be a 2.5 Kb mRNA that contains the four genes. A less abundant transcript containing only the L10 and L12 gene was also detected. Upstream of the transcription initiation sites, sequences that match the consensus sequence for archaebacterial promoters (TTTAT/AA) were found. Transcription termination sites were located within or after pyrimidine rich regions. Three of the ribosomal protein genes start with unusual initiation codons, GTG in the case of the L1 and L10 genes and TTG in the case of the L11 gene. Putative Shine Dalgarno sequences, complementary to the 3' end of Sulfolobus 16S rRNA, were detected in the region surrounding the initiation codon. In some cases (L1 and L10 genes), the initiation codon was found to be part of this sequence. Sequence comparison of the ribosomal proteins from Sulfolobus with those from other organisms, revealed that the Sulfolobus sequences are closer to those from other archaebacteria, thus supporting the existence of the archaebacterial kingdom. Comparison of the sequences of the L10 and L12 proteins from the three kingdoms revealed that the archaebacterial sequences are closer to the eukaryotes. / Graduate