Secreted RNase activity of wild-type vs srnA mutants of L. pneumophila. Cell-free supernatants from cultures of wild-type strain 130b, lspF mutant NU275, srnA mutants NU328 and NU329, lpg2847 mutant NU332, and srnA lpg2847 double mutant NU333 were incubated with yeast RNA, and the release of nucleotides was measured by the increase in A260 over a 40 min period. A BYE medium control was also included. The values presented are the means±SD obtained from duplicate samples and are representative of at least four independent experiments. The reduced levels of activity seen for the lspF, srnA and srnA lpg2847 mutants relative to wild-type were statistically significant (P<0.05; Student's t-test).