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Mycoplasma genitalium is a human pathogen that mediates cell adhesion by a complex structure known as the attachment organelle. This structure is composed of cytadhesins and cytadherence-associated proteins, but few data are available about the specific role of these proteins in M. genitalium cytadherence. We have deleted by homologous recombination the mg191 and mg192 genes from the MgPa operon encoding the P140 and P110 cytadhesins. Molecular characterization of these mutants has revealed a reciprocal posttranslational stabilization between the two proteins. Loss of either P140 or P110 yields a hemadsorption-negative phenotype and correlates with decreased or increased levels of cytoskeleton-related proteins MG386 and DnaK, respectively. Scanning electron microscopy analysis reveals the absolute requirement of P140 and P110 for the proper development of the attachment organelle. The phenotype described for these mutants resembles that of the spontaneous class I and class II cytadherence-negative mutants [G. R. Mernaugh, S. F. Dallo, S. C. Holt, and J. B. Baseman, Clin. Infect. Dis. 17(Suppl. 1):S69-S78, 1993], whose genetic basis remained undetermined until now. Complementation assays and sequencing analysis demonstrate that class I and class II mutants are the consequence of large deletions affecting the mg192 and mg191-mg192 genes, respectively. These deletions originated from single-recombination events involving sequences of the MgPa operon and the MgPa island located immediately downstream. We also demonstrate the translocation of MgPa sequences to a particular MgPa island by double-crossover events. Based on these observations, we propose that in addition to being a source of antigenic variation, MgPa islands could be also involved in a general phase variation mechanism switching on and off, in a reversible or irreversible way, the adhesion properties of M. genitalium.Mycoplasmas are microorganisms derived from gram-positive bacteria with low GϩC content, and they are characterized by streamlined genomes and the absence of a cell wall. Their reduced metabolic abilities (32) lead them to a parasitic lifestyle, and many of them are pathogens for humans and animals (5). In particular, Mycoplasma genitalium is the leading cause of chlamydia-negative, nongonococcal urethritis (13), and it has been also associated with several extragenitourinary diseases (43). With just 517 genes (9), M. genitalium is considered one of the smallest self-replicating cells, and it is currently used as a model for the studies of the minimal gene complement (11,14). Despite its apparent simplicity, M. genitalium adheres to host cells via its complex tip structure, known as the attachment or terminal organelle. This polar membrane extension confers a flask-shaped appearance to the M. genitalium cells, and it is characterized by an electron-dense core that is a part of the mycoplasma cytoskeleton. The terminal organelle is present in several Mycoplasma species, and it has been extensively studied in Mycoplasma pneumoniae. In the lat...

The thermal stability of proteins was studied, 195 single amino acid residue replacements reported elsewhere being analysed for several protein conformational characteristics: type of residue replacement; conservative versus nonconservative substitution; replacement being in a homologous stretch of amino acid residues; change in hydrogen bond, van der Waals and secondary structure propensities; solvent-accessible versus inaccessible replacement; type of secondary structure involved in the substitution; the physico-chemical characteristics to which the thermostability enhancement can be attributed; and the relationship of the replacement site to the folding intermediates of the protein, when known. From the above analyses, some general rules arise which suggest where amino acid substitutions can be made to enhance protein thermostability: substitutions are conservative according to the Dayhoff matrix; mainly occur on conserved stretches of residues; preferentially occur on solvent-accessible residues; maintain or enhance the secondary structure propensity upon substitution; contribute to neutralize the dipole moment of the caps of helices and strands; and tend to increase the number of potential hydrogen bonding or van der Waals contacts or improve hydrophobic packing.