Rickettsia prowazekii transports UMP and GMP, but not CMP, as building blocks for RNA synthesis

Rickettsia prowazekii, the etiological agent of epidemic typhus, is an obli gate intracellular bacterium and is apparently unable to synthesize ribonuc leotides de novo, Here, we show that as an alternative, isolated, purified R. prowazekii organisms transported exogenous uridyl- and guanylribonucleot ides and incorporated these labeled precursors into their RNA in a rifampin -sensitive manner. Transport systems for nucleotides,which we have shown pr eviously and show here are present in rickettsiae, have never been reported in free-living bacteria, and the usual nucleobase and nucleoside transport systems are absent in rickettsiae. There was a clear preference for the mo nophosphate form of ribonucleotides as the transported substrate. In contra st, rickettsiae did not transport cytidylribonucleotides. The source of ric kettsial CTP appears to be the transport of UMP followed by its phosphoryla tion and the amination of intrarickettsial UTP to CTP by CTP synthetase, A complete schema of nucleotide metabolism in rickettsiae is presented that i s based on a combination of biochemical, physiological, and genetic informa tion.