Essential genes of a minimal bacterium

Abstract

Mycoplasma genitalium has the smallest genome of any organism that can be grown in pure culture. It has a minimal metabolism and little genomic redundancy. Consequently, its genome is expected to be a close approximation to the minimal set of genes needed to sustain bacterial life. Using global transposon mutagenesis, we isolated and characterized gene disruption mutants for 100 different nonessential protein-coding genes. None of the 43 RNA-coding genes were disrupted. Herein, we identify 382 of the 482 M. genitalium protein-coding genes as essential, plus five sets of disrupted genes that encode proteins with potentially redundant essential functions, such as phosphate transport. Genes encoding proteins of unknown function constitute 28% of the essential protein-coding genes set. Disruption of some genes accelerated M. genitalium growth.

Fig. 1.

Accumulation of new disrupted M. genitalium genes (red) and new transposon insertion sites in the genome (black) as a function of the total number of analyzed primary colonies and subcolonies with insertion sites different from that of the parental primary clone.

Fig. 2.

Global transposon mutagenesis of M. genitalium. The locations of transposon insertions from the current study are noted by a Δ below the insertion site on the map. Insertions mapped in our previous study (4) are noted with a ▿. Sites with 10 or more insertions are noted by a red filled triangle (▴).

Fig. 3.

Metabolic pathways and substrate transport mechanisms encoded by M. genitalium. Metabolic products are colored red, and mycoplasma proteins are black. White letters on black boxes mark nonessential functions or proteins based on our current gene disruption study. Question marks denote enzymes or transporters not identified that would be necessary to complete pathways, and those missing enzyme and transporter names are colored green. Transporters are colored according to their substrates: yellow, cations; green, anions and amino acids; orange, carbohydrates; purple, multidrug and metabolic end product efflux. The arrows indicate the predicted direction of substrate transport. The ABC type transporters are drawn as follows: rectangle, substrate-binding protein; diamonds, membrane-spanning permeases; circles, ATP-binding subunits.