Key role of amino acid residues in the dimerization and catalytic activation of the autolysin LytA, an important virulence factor in Streptococcus pneumoniae

LytA, the main autolysin of Streptococcus pneumoniae, was the first member of the bacterial N-acetylmuramoyl-l-alanine amidase (NAM-amidase) family of proteins to be well characterized. This autolysin degrades the peptidoglycan bonds of pneumococcal cell walls after anchoring to the choline residues of the cell wall teichoic acids via its choline-binding module (ChBM). The latter is composed of seven repeats (ChBRs) of approximately 20 amino acid residues. The translation product of the lytA gene is the low-activity E-form of LytA (a monomer), which can be "converted" (activated) in vitro by choline into the fully active C-form at low temperature. The C-form is a homodimer with a boomerang-like shape. To study the structural requirements for the monomer-to-dimer modification and to clarify whether "conversion" is synonymous with dimerization, the biochemical consequences of replacing four key amino acid residues of ChBR6 and ChBR7 (the repeats involved in dimer formation) were determined. The results obtained with a collection of 21 mutated NAM-amidases indicate that Ile-315 is a key amino acid residue in both LytA activity and folding. Amino acids with a marginal position in the solenoid structure of the ChBM were of minor influence in dimer stability; neither the size, polarity, nor aromatic nature of the replacement amino acids affected LytA activity. In contrast, truncated proteins were drastically impaired in their activity and conversion capacity. The results indicate that dimerization and conversion are different processes, but they do not answer the questions of whether conversion can only be achieved after a dimer formation step.

Biofilm formation by Streptococcus pneumoniae: role of choline, extracellular DNA, and capsular polysaccharide in microbial accretion

Streptococcus pneumoniae colonizes the human upper respiratory tract, and this asymptomatic colonization is known to precede pneumococcal disease. In this report, chemically defined and semisynthetic media were used to identify the initial steps of biofilm formation by pneumococcus during growth on abiotic surfaces such as polystyrene or glass. Unencapsulated pneumococci adhered to abiotic surfaces and formed a three-dimensional structure about 25 microm deep, as observed by confocal laser scanning microscopy and low-temperature scanning electron microscopy. Choline residues of cell wall teichoic acids were found to play a fundamental role in pneumococcal biofilm development. The role in biofilm formation of choline-binding proteins, which anchor to the teichoic acids of the cell envelope, was determined using unambiguously characterized mutants. The results showed that LytA amidase, LytC lysozyme, LytB glucosaminidase, CbpA adhesin, PcpA putative adhesin, and PspA (pneumococcal surface protein A) mutants had a decreased capacity to form biofilms, whereas no such reduction was observed in Pce phosphocholinesterase or CbpD putative amidase mutants. Moreover, encapsulated, clinical pneumococcal isolates were impaired in their capacity to form biofilms. In addition, a role for extracellular DNA and proteins in the establishment of S. pneumoniae biofilms was demonstrated. Taken together, these observations provide information on conditions that favor the sessile mode of growth by S. pneumoniae. The experimental approach described here should facilitate the study of bacterial genes that are required for biofilm formation. Those results, in turn, may provide insight into strategies to prevent pneumococcal colonization of its human host.

Molecular peculiarities of the lytA gene isolated from clinical pneumococcal strains that are bile insoluble

The autolytic LytA amidase from 12 bile (deoxycholate)-insoluble streptococcal isolates (formerly classified as atypical Streptococcus pneumoniae) showing different antibiotic resistance patterns was studied. These atypical strains, which autolyze at the end of the stationary phase of growth, contain highly divergent lytA alleles (pairwise evolutionary distances of about 20%) compared to the lytA alleles of typical pneumococci. The atypical LytA amidases exhibit a peculiar deletion of two amino acids responsible for cell wall anchoring in the carboxy-terminal domain and have a reduced specific activity. These enzymes were inhibited by 1% deoxycholate but were activated by 1% Triton X-100, a detergent that could be used as an alternative diagnostic test for this kind of strain. Preparation of functional chimeric enzymes, PCR mutagenesis, and gene replacements demonstrated that the characteristic bile insolubility of these atypical strains was due to their peculiar carboxy-terminal domain and that the 2-amino-acid deletion was responsible for the inhibitory effect of deoxycholate. However, the deletion alone did not affect the specific activity of LytA. A detailed characterization of the genes encoding the 16S rRNA and SodA together with multilocus sequence typing indicated that the strains studied here are not a single clone and, although they cannot be strictly classified as typical pneumococci, they represent a quite diverse pool of organisms closely related to S. pneumoniae. The clinical importance of these findings is underlined by the role of the lytA gene in shaping the course of pneumococcal diseases. This study can also contribute to solving diagnostic problems and to understanding the evolution and pathogenic potential of species of the Streptococcus mitis group.

Structural analysis and biological significance of the cell wall lytic enzymes of Streptococcus pneumoniae and its bacteriophage

The development of an appropriate technique for the identification of autolysin-defective mutants of pneumococcus has been a fundamental step to carry out studies on the molecular characteristics of the lytic enzymes of Streptococcus pneumoniae and its bacteriophage. Our results show that the principal pneumococcal autolysin (an amidase) is responsible for the separation of the daughter cells at the end of the cell division. On the other hand, this system provides a reliable experimental model to support the extended idea concerning the modular organization of most proteins. The comparative analyses of the deduced amino acid sequences of these enzymes, as well as the construction of functional chimeric phage-bacterial enzymes, demonstrate that the C-terminal domain, which contains a large number of repeated amino acid motifs, is the substrate-binding domain, whereas the N-terminal domain provides enzymatic specificity. We propose that the pneumococcal lytic enzymes have evolved by modular exchange providing examples of the types of novel genes that the bacteria or the phage might create to allow them to become adapted to new environmental situations.

Overproduction and rapid purification of the amidase of Streptococcus pneumoniae

Oligonucleotide-directed mutagenesis of a plasmid containing the lytA gene coding for the pneumococcal amidase has allowed the separation of the coding sequence of the gene. This sequence has been placed in plasmid pIN-III(lppP-5)-A3 downstream from both a modified lipoprotein promoter and the lactose promoter to construct the recombinant plasmid pGL100. When Escherichia coli RB 791 (pGL100) was grown in the presence of lactose, the pneumococcal amidase accounted for 7% of the total protein present in this strain after 18 h incubation at 37 degrees C. The overproduced amidase was purified in a single-step procedure using a choline-Sepharose 6B column taking advantage of the fact that this enzyme was the unique protein with affinity for choline present in extracts obtained from E. coli RB791 (pGL100). The development of the above design opens up the possibility of studying the mechanism that regulates the activity of this important autolysin by using physiochemical techniques that require the availability of high amounts of purified amidase.