Microarray expression analysis comparing mRNA levels
between wild-type and 6S RNA null cells (ssrS3) demonstrated
that expression of many genes was altered in a 6S
RNA-dependent manner (A. T. Cavanagh & K. M.
Wassarman, unpublished data). One mRNA that increased
in the absence of 6S RNA was relA, which encodes ppGpp
synthase I. Specifically, in cells lacking 6S RNA, relA mRNA
was increased 2.3-fold relative to wild-type cells in late
stationary phase (16 h of growth in RDM). Given the
importance of ppGpp in regulating gene expression in
response to nutrient limitation, a time when 6S RNA
function is also important, we examined the relationship
between 6S RNA function, relA expression and ppGpp
levels. Expression of spoT, which encodes the bifunctional
SpoT enzyme that can synthesize and degrade ppGpp
(Magnusson et al., 2005; Potrykus & Cashel, 2008), was relatively unchanged (0.85 in cells lacking 6S RNA
compared with wild-type cells).