THE DISTANCE BETWEEN BACTERIAL SPECIES IN SEQUENCE SPACE

Despite the revolution caused by information from macromolecular seque nces, the basis of bacterial classification remains the genus and the species. How do these terms relate to the variety of bacteria that exi st on earth? In this paper, the inter- and intraspecies differences in amino acid sequence of several bacterial electron transport proteins, cytochromes c, and blue copper proteins are compared. For the soil an d water organisms studied, bacterial species can be classed as ''tight '' when there is little intraspecies variation, or ''loose'' when this variation is large. For this set of proteins and organisms, interspec ies variation is much larger than that within a species. Examples of ' 'tight'' species are Pseudomonas aeruginosa and Rhodobacter sphaeroide s, while Pseudomonas stutzeri and Rhodopseudomonas palustris are loose species. The results are discussed in the context of the origin and a ge of bacterial species, and the distribution of genomes in ''sequence space.'' The situation is probably different for commensal or pathoge nic bacteria, whose population structure and evolution are linked to t he properties of another organism.