Temperature related alterations in the acidic alanine-rich "A" protein from the 50S ribosomal particle of the extreme halophile, Halobacterium cutirubrum

Complete nucleotide sequence of the ribosomal 'A' protein operon from the archaebacterium, Halobacterium halobium

DNA fragments cloned from the halophilic archaebacterium Halobacterium halobium including the gene for ribosomal 'A' protein (HhaL20) were sequenced. Sequence and transcript analyzes indicated that the 'A' protein message (ORFC) was co-transcribed with two neighbouring open reading frames (ORFA and B) as a single unit. Comparison of the amino acid sequences deduced from ORFA and B with those from other organisms demonstrated that ORFA encoded the protein homologous to the ribosomal protein L8 of Halobacterium cutirubrum (HcuL8) and L1 of Escherichia coli (EL1), while ORFB encoded the protein homologous with the human ribosomal protein P0. The sequences which were able to facilitate mRNA.16S-rRNA hybridization were found about 10-20 nucleotides upstream of the initiation codon ATG of each of the three ORFs. The 5'- and 3'-ends of the operon were mapped with nuclease S1. A putative promoter (A+T-rich) and termination signal (T-rich) were found to be present within the 5'- and 3'-flanking sequence.

Chemistry and biology of E. coli ribosomal protein L12

E. coli ribosomal protein L12, because of its unique features, has been studied in more detail than perhaps any of the other ribosomal proteins. Unlike the other ribosomal proteins that are generally present in stoichiometric amounts, there are four copies of L12 per ribosome, some of which are acetylated on the N-terminal serine. The acetylated species, referred to as L7, has not been shown, as yet, to possess any different biological activity than L12. A specific enzyme that acetylates L12 to form L7, using acetyl-CoA as the acetyl donor, has been purified from E. coli extracts. L12 is also unique in that it does not contain cysteine, tryptophan, histidine, or tyrosine, is very acidic (pI: 4.85) and has a high content of ordered secondary structure (approximately 50%). The protein is normally found in solution as a dimer and also forms a tight complex with ribosomal protein L10. There are three methionine residues in L12, located in the N-terminal region of the protein, one or more of which are essential for biological activity. Oxidation of the methionines to methionine sulfoxide prevents dimer formation and inactivates the protein. The four copies of L12 are located in the crest region(s) of the 50S ribosomal subunit. There is good evidence that the soluble factors, such as IF-2, EF-Tu, EF-G and RF, interact with L12 on the ribosome during the process of protein synthesis. This interaction is essential for the proper functioning of each of the factors and for GTP hydrolysis associated with the individual partial reactions of protein synthesis. The L12 gene is located on an operon that contains the genes for L10 and beta beta' subunits of RNA polymerase at about 88 min on the bacterial chromosome. DNA-directed in vitro systems have been used to study the unique regulation of the expression of these genes. Autogenous regulation, translational control, and transcription attenuation are regulatory mechanisms that function to control the synthesis of these proteins.

The N-terminal sequence of the ribosomal 'A' protein from two moderate halophiles, Vibrio costicola and an unidentified moderate (NRCC 11227)

The 'A' protein, equivalent to ribosomal protein EL7/L12 from Escherichia coli, has been isolated and purified from two moderate halophiles Vibrio costicola and NRCC 11227. The 'A' protein from V. costicola contained an N-terminal serine and separated into two forms on DEAE-cellulose and two-dimensional electrophoresis while the equivalent protein in NRCC 11227 contained an N-terminal alanine residue and was present in only one form. The amino acid composition and mobility on two-dimensional gels indicated these proteins were very similar to EL7/L12. The first 38 residues of the 'A' proteins were sequenced and compared to the equivalent protein from E. coli and the extreme halophile Halobacterium cutirubrum. The N-terminal region of the 'A' protein from both moderate halophiles showed substantial homology to EL 12 (75--80%) but no evidence of any homology to the equivalent protein from the extreme halophile. The ribosomal proteins equivalent to ES1A in E. coli were also isolated and their amino acid compositions determined.

Topography of Escherichia coli ribosomal protein L12 in situ

The ionization constants of each individual free amino group of Escherichia coli ribosomal protein L12 as it exists on the native intact 50-S subunit has been determined. All these amino groups are exposed and have pK values varying from 9.7 to 11.0. The amino terminus, on the other hand, is unreactive and therefore appears to be buried. In addition, the reactivities indicate that the aminoterminal region of residues 1--59 undergoes a pH-dependent conformational change on the 50-S subunit.