β-Galactosidase activity due to the core 16S/23S rRNA gene promoter in Sulfolobus was 1.7–3-fold lower in the stationary phase than in exponential growth (Fig. 3). The pattern of β-galactosidase activity did not change significantly when normalized for

the absolute copy number of the lacS gene by qPCR, indicating that the increase in activity in exponential growth Ceritinib datasheet was due to regulation of the 16S/23S rRNA gene promoter, not gene dosage (Fig. 3b). The 42-bp 16S/23S rRNA gene core promoter is the smallest reported regulated promoter for Sulfolobus. These findings are consistent with evidence of upregulation of rRNA transcription during exponential growth in E. coli and Saccharomyces cerevisiae (yeast) (Nomura, 1999) and with microarray data from halophilic archaea showing that ribosomal protein gene transcription is higher during exponential growth than in the stationary phase (Lange et al., 2007). Moreover, rRNA in crude preparations from Natronococcus occultus decreases in the stationary phase (Nercessian

& Conde, 2006). The mechanism for core rRNA promoter regulation in S. solfataricus is obscure. The decrease in β-galactosidase activity observed during the stationary phase may be due to growth rate-dependent transcriptional regulation or stringent control in response to decreasing nutrient availability and/or charged tRNAs. The latter has been learn more shown to decrease total stable RNA accumulation in Sulfolobus (Cellini et al., 2004). As in E. coli and yeast, it is likely that there are multiple mechanisms contributing to regulation of the Sulfolobus 16S/23S rRNA gene operon. There is considerable

evidence that archaeal transcriptional regulators interact with core promoters, either binding between or overlapping the TATA box and the transcriptional start site (Peng et al., 2011). In vivo and in vitro analyses have determined several regulatory regions and the start site of the 16S/23S rRNA gene in S. shibatae LY294002 (Hudepohl et al., 1990; Reiter et al., 1990; Hain et al., 1992; Qureshi et al., 1997). The core promoter sequences necessary for transcription initiation in vitro are between −38 and −2 bases relative to the transcription start, identical to those used here in vivo. This region encompasses the proximal promoter element (PPE) (an AT-rich sequence −11 to −2 conserved in Sulfolobus stable RNA promoters), the TATA box, and several bases upstream thereof (Reiter et al., 1990), later identified as a transcription factor B (TFB) recognition element (BRE) (Qureshi & Jackson, 1998). A weak positive regulatory region between −354 and −190 and a negative regulatory region between −93 and −38 were also found (Reiter et al., 1990).