Global analysis of proteins synthesized by Mycobacterium smegmatis provides direct evidence for physiological heterogeneity in stationary-phase cultures.

Abstract

We have characterized the induction kinetics of approximately 1,700 proteins during entry into and survival in carbon-starved stationary phase by Mycobacterium smegmatis. Strikingly, among the patterns of expression observed were a group of proteins that were expressed in exponential-phase cultures and severely repressed in 48-h stationary-phase cultures (Spr or stationary-phase-repressed proteins) but were synthesized again at high levels in > or =128-day stationary-phase cultures (Spr(128) proteins). A number of Spr(128) proteins were identified, and they included the heat shock protein DnaK, the tricarboxylic acid cycle enzyme succinyl coenzyme A synthase, a FixA-like flavoprotein, a single-stranded DNA binding protein, and elongation factor Tu (EF-Tu). The identification of EF-Tu as an Spr(128) protein is significant, as ribosomal components are known to be expressed in a growth rate-dependent way. We interpreted these data in terms of a model whereby stationary-phase mycobacteria comprise populations of cells that differ in both their growth status and gene expression patterns. To investigate this further, we constructed gene fusions between the rpsL gene promoter (which heads the Mycobacterium smegmatis operon encoding the tuf gene encoding EF-Tu) or the rrnA promoter gene and an unstable variant of green fluorescent protein. While the majority of cells in old stationary-phase cultures had low levels of fluorescence and so rpsL expression, a small but consistently observed population of approximately 1 in 1,000 cells was highly fluorescent. This indicates that a small fraction of the cells was expressing rpsL at high levels, and we argue that this represents the growing subpopulation of cells in stationary-phase cultures.