Role of mRNA Stability in Growth Phase Regulation of Gene Expression in the Group A Streptococcus

Abstract

The impressive disease spectrum of Streptococcus pyogenes (the group A streptococcus [GAS]) is believed to be determined by its ability to modify gene expression in response to environmental stimuli. Virulence gene expression is controlled tightly by several different transcriptional regulators in this organism. In addition, expression of most, if not all, GAS genes is determined by a global mechanism dependent on growth phase. To begin an analysis of growth-phase regulation, we compared the transcriptome 2 h into stationary phase to that in late exponential phase of a serotype M3 GAS strain. We identified the arc transcript as more abundant in stationary phase in addition to the sag and sda transcripts that had been previously identified. We found that in stationary phase, the stability of sagA, sda, and arcT transcripts increased dramatically. We found that polynucleotide phosphorylase (PNPase [encoded by pnpA]) is rate limiting for decay of sagA and sda transcripts in late exponential phase, since the stability of these mRNAs was greater in a pnpA mutant, while stability of control mRNAs was unaffected by this mutation. Complementation restored the wild-type decay rate. Furthermore, in a pnpA mutant, the sagA mRNA appeared to be full length, as determined by Northern hybridization. It seems likely that mRNAs abundant in stationary phase are insensitive to the normal decay enzyme(s) and instead require PNPase for this process. It is possible that PNPase activity is limited in stationary phase, allowing persistence of these important virulence factor transcripts at this phase of growth.

RNase Y-mediated regulation of the streptococcal pyrogenic exotoxin B

“…For instance, a group of transcripts encoding virulence proteins ( e.g . Streptolysin S and Streptodornase D) are highly stable during stationary growth phase as their degradation – by RNases J1/J2 and PNPase – is delayed compared to other mRNAs [41,42]. It was proposed that the differences in RNA decay initiation of these two groups of mRNAs rely on features ( e.g .…”

The influence of Hfq and ribonucleases on the stability of the small non-coding RNA OxyS and its targetrpoSinE. coliis growth phase dependent

“…A growth-phase dependent decay of the sagA mRNA in Streptococcus pyogenes could be attributed to altered processing by PNPase. 36 A major role of PNPase in the turn-over of several small RNAs that control expression of outer membrane proteins was demonstrated in E. coli. 37 In general little is known to date on different levels or activities of RNases or their different organization in protein complexes in different growth phases.…”

Regulation of virulence gene expression inStreptococcus pyogenes

“…We learned that rnr (which codes for RNase R, a processive 3'-to-5' exoribonuclease that cleaves structured mRNA) 11,12 and yhaM (encoding YhaM, a 3'-to-5' exoribonuclease present only in Gram + bacteria) 13 have little effect on decay of Class II mRNAs, but deletion of pnpA, the gene encoding polynucleotide phosphorylase (PNPase, one of the major 5'-to-3' exonucleases that affects global mRNA turnover), 12 decreases the decay rate for Class II messages without affecting the time at which decay is initiated. 9 This suggests that the 3'-to-5' exoribonuclease activity of PNPase is rate-limiting for decay of Class II mRNAs, but that a different enzyme initiates decay of these transcripts. In addition, deletion of pnpA has little effect on decay of Class I messages, indicating that PNPase is involved in distinguishing between messages of the two Classes.…”

“…9 We called these Class I messages. However, transcripts of a few genes are present in greater quantity in stationary than in exponential phase.…”

“…We found that all of these messages, which we designated Class II, have extremely long half-lives (≥100 min) at two hours into stationary phase, as determined by RNase protection and northern blot analyses. 9 To distinguish the effect of transcription initiation from that of mRNA decay on the amount of a Class I transcript in stationary phase, we expressed has (Class I) from the promoter Psag, which normally produces a Class II message. The has message expressed from Psag decayed rapidly in stationary phase (half life 7 min), indicating that decay rate is a major determinant of growth phase regulation and that decay rate underlies the distinction between the two mRNA Classes.…”