Transcription in Mycoplasma pneumoniae

Abstract

Bacteria have developed a broad variety of mechanisms to react to changes in their environment. All mechanisms have in common that an external stimulus is sensed and transduced into a cellular response, either adjustments of enzymatic or physiological activity or alteration of gene expression, aiming to adapt to these new conditions. The stimuli sensed, as well as the output responses, can both be specific or general. As an example, the zinc uptake repressor Zur in the soil bacterium Bacillus subtilis specifically senses intracellular Zn 2+ concentrations and mediates specific responses towards zinc starvation, whereas the expression and activity of general stress-response sigma factor SigS in Escherichia coli is triggered by various signals (Gaballa and Helmann, 1998;Hengge-Aronis, 2002). Cellular recognize and bind to specific promoter sequences (Gruber and Gross, 2003). Often sigma factors themselves are targets of regulatory events, either on the level of expression like B. subtilis SigD (Mirel et al., 2000), or by inactivation through interactions with so called anti-sigma factors. Such anti-sigma factors can either be proteins like FlgM in the case of B. subtilis SigD, or other molecules like 6S RNA in E. coli (Fredrick and Helmann, 1996;Trotochaud and Wassarman, 2004). Besides sigma and anti-sigma factors, other proteins or molecules can also trigger the affinity of RNA polymerase towards specific promoters. al., 2007). Based on their bi-functional role as enzymes and transcription regulators, these proteins are referred to as ´trigger enzymes´.RNA switches are cis-acting regulatory RNA elements that control transcription or translation by anti-termination. They are located in the 5´leader sequence of transcripts and form a terminator structure in the absence of their effector, leading to transcription breakup. If the Post-transcriptional regulation in bacteriaFor a long period, post-transcriptional regulations where thought to occur only by RNA processing or degradation. However, during the last years, another widely distributed mechanism of post-transcriptional control has been discovered involving small, non-coding RNAs (sRNA). These sRNAs can be grouped in two categories, according to their mechanism of regulation: (I) small, trans-acting antisense sRNAs that bind to their target mRNA by base pairing and (II) sRNAs that interact with proteins, thereby triggering their activity. An example of group (II) is E. coli 6S RNA, described previously. sRNAs of group (I) can either effect translation initiation by masking or exposing the Shine-Dalgarno sequence of its target mRNAs (e.g. B. subtilis SR1) or by altering the stability of their targets as for Staphylococcus aureus RNAIII (Heidrich et al., 2006;Huntzinger et al., 2005). Moreover, recent publications showed that also small mRNAs could function as regulatory RNAs, and that titration effects trigger regulatory effects of sRNA with multiple targets (