Obligate intracellular parasites: Rickettsia prowazekii and Chlamydia trachomatis

Abstract

Transitions to obligate intracellular parasitism have occurred at numerous times in the evolutionary past. The genome sequences of two obligate intracellular parasites, Rickettsia prowazekii and Chlamydia trachomatis, were published last year. A comparative analysis of these two genomes has revealed examples of reductive convergent evolution, such as a massive loss of genes involved in biosynthetic functions. In addition, both genomes were found to encode transport systems for ATP and ADP, not otherwise found in bacteria. Here, we discuss adaptations to intracellular habitats by comparing the information obtained from the recently published genome sequences of R. prowazekii and C. trachomatis.z 1999 Federation of European Biochemical Societies.

Lipoic Acid Metabolism in Microbial Pathogens

“…C. trachomatis is an obligate intracellular pathogen and is similar in this respect to the Rickettsia Alphaproteobacteria described above. Although C. trachomatis and R. prowazekii are not phylogenetically related, the contents of their genomes are surprisingly similar, perhaps due to the convergent evolution of both obligate intracellular pathogens (263). Both bacteria contain PDH gene clusters encoding the E1␣, E1␤, and E2 subunits, similar to those found in Grampositive bacteria (Table 2).…”

Closing in on Chlamydia and its intracellular bag of tricks

“…Extensive comparisons of the Chlamydia genomes among themselves and with those of other organisms have recently been published (Kalman et al, 1999 ;Makarova et al, 2000 ;Read et al, 2000 ;Stephens et al, 1998 ;Zomorodipour & Andersson, 1999). From these, a global understanding of chlamydial metabolic and biosynthetic capacities (McClarty, 1999 ;Stephens et al, 1998) has been the most tangible benefit.…”

Size of the protein-coding genome and rate of molecular evolution

“…The expected dependence of n on Ne and mutation rate has been supported by observation of reduced n in chronic pathogens and symbionts, which may experience small Ne due to bottlenecks during infection of hosts Andersson and Kurland 1998;Moran 1996;Zomorodipour and Andersson 1999) and higher per-site mutation rates (Ochman et al 1999). Thus, in bacteria and viruses with small Ne and high mutation rates, the selection required to maintain a given genome size increases and should become visible since the genome size and organization are more evolutionarily labile than gene sequences (Huyen and Bork 1998).…”