Upon entry into stationary phase, on the other hand, cAMP-CRP may upregulate "rpoS" transcription (Hengge-Aronis, 2002). While these observations may explain the seemingly dual nature of the cAMP-CRP binding sites, they require an explanation of phase-dependent selection of cAMP-CRP site activation to fully account for the contradictory data. Additional regulatory controls for "rpoS" transcription include: BarA, a Histidine sensor kinase which can activate OmpR and thereby promote porin synthesis; levels of small molecules such as ppGppp which may hinder transcriptional elongation or stability in response to amino acid limitation, or carbon, nitrogen or phosphorus starvation (Gentry "et al.", 1993). Despite the numerous controls on "rpoS" transcription, cellular "rpoS" mRNA levels remain high during exponential phase and the majority of extracellular stimuli do not significantly affect "rpoS" transcription. Most RpoS expression is determined at the translational level. sRNAs (small noncoding RNAs) sense environmental changes and in turn increase "rpoS" mRNA translation to allow the cell to accordingly adjust to external stress. The promoter of the 85 nucleotide sRNA DsrA contains a temperature-sensitive transcription initiation thermocontrol as it is repressed at high (42˚C) temperatures, but induces (perhaps by complementary binding to) "rpoS" at low (25˚C) temperatures. Another sRNA, RprA, stimulates "rpoS" translation in response to cell surface stress signaled via the RcsC sensor kinase. A third type of sRNA, OxyS, is regulated by OxyR, the primary sensor of oxidative shock. The mechanism by which OxyS interferes with "rpoS" mRNA translational efficiency is not known. However, the RNA-binding protein Hfq is implicated in the process. Hfq binds to "rpoS" mRNA "in vitro" and may thereby modify "rpoS" mRNA structure for optimal translation. Hfq activates both DsrA and RprA. In contrast, LeuO inhibits "rpoS" translation by repressing "dsrA" expression and the histone-like protein HN-S (and its paralog StpA) inhibits "rpoS" translation via an unknown mechanism. In addition, H-NS, LeuO, Hfq and DsrA form an interconnected regulatory network that ultimately controls "rpoS" translation.