There have been some surprises arising from chlamydial genomic sequencing that have substantially altered pre-existing ideas concerning these organisms.

• Surprise 1. It had previously been thought that chlamydiae are "energy parasites", requiring energy from host cells in the form of the high-energy biochemical, ATP (adenosine tri-phosphate). Indeed both C. trachomatis and C. pneumoniae have evolutionarily related genes encoding proteins (ADP / ATP translocases) whose function is to transport ATP from the host cell cytoplasm. This, it was thought, kept chlamydial replication in step with that of the host cell. The finding of a constellation of genes encoding metabolic pathways for the chlamydial production of ATP therefore came as a complete surprise.
• Surprise 2. Previous laboratory-based studies had indicated that chlamydiae lack the net-like compound, peptidoglycan, that serves to strengthen bacterial cell walls rather like steel strengthens a motorcar tire. This, it was thought, might explain why chlamydiae, although they have penicillin-binding proteins in their cell wall, were only partially sensitive to penicillin. However genome sequencing revealed that C. trachomatis and C. pneumoniae have the complete gene complement for the synthesis of peptidoglycan, even though the amount actually synthesized in EB appears to be very small (see peptidoglycan).
• Surprise 3. The chlamydial sequences lacked the FtsZ gene (pronounced FitsZee) encoding a protein essential for cell division that is found in all other sequenced bacterial genomes. This might be a reflection of the small amount of peptidoglycan in chlamydiae (see peptidoglycan).
• Surprise 4. Although a new surface protein had been identified in C. abortus [Longbottom et al., 1998] and subsequently in C. pneumoniae [Knudsen et al., 1999], the existence of a whole superfamily of these proteins was unexpected [see: polymorphic membrane proteins].
• Surprise 5. In C. trachomatis but not C. pneumoniae a family of genes encoding proteins with phospholipase D activities were identified. It was thought that these might be related to the ability of chlamydial containing inclusions to avoid destruction by lysosomes, or to the ability of chlamydiae to modify host cell lipid (fat).
• Surprise 6. The unexpectedly high number of chlamydial genes related to higher organisms, particularly plant chloroplasts. See Plant Chlamydia relationships.

[MEW] Jan 2003

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