RRNA operon copy number reflects ecological strategies of bacteria

Although natural selection appears to favor the elimination of gene redunda ncy in prokaryotes, multiple copies of each rRNA-encoding gene are common o n bacterial chromosomes. Despite this conspicuous deviation from single-cop y genes, no phenotype has been consistently associated with rRNA gene copy number. We found that the number of rRNA genes correlates with the rate at which phylogenetically diverse bacteria respond to resource availability, S oil bacteria that formed colonies rapidly upon exposure to a nutritionally complex medium contained an average of 5.5 copies of the small subunit rRNA gene, whereas bacteria that responded slowly contained an average of 1.4 c opies. In soil microcosms pulsed with the herbicide 2, 4-dichlorophenoxy-ac etic acid (2,4-D), indigenous populations of 2,4-D-degrading bacteria with multiple rRNA genes ((x) over bar = 5.4) became dominant, whereas populatio ns with fewer rRNA genes ((x) over bar = 2.7) were favored in unamended con trols. These findings demonstrate phenotypic effects associated with rRNA g ene copy number that are indicative of ecological strategies influencing th e structure of natural microbial communities.