6S RNA: A Regulator of Transcription

Abstract

6S RNA was first identified in total RNA from E. coli due to its high abundance [Hindley, 1967]. It was one of the first small RNAs to be sequenced [Brownlee, 1971] and it was shown to be quite stable [Lee et al., 1978]. In contrast to many other stable RNAs, its accumulation is not subject to stringent response [Ikemura and Dahlberg, 1973]. Limited sequence and structural homology to 7SL RNA led to an early hypothesis that 6S RNA was a bacterial signal recognition particle RNA (7SL or SRP RNA) involved in protein transport in eukaryotes [Walter and Blobel, 1983]. This hypothesis turned out to be incorrect as cells lacking 6S RNA did not exhibit protein secretion defects [Lee et al., 1985] and 4.5S RNA was later identified as the bacterial SRP RNA [Poritz et al., 1990; Ribes et al., 1990]. Nevertheless, these studies demonstrated that 6S RNA, encoded by the ssrS gene, is dispensable for growth [Lee et al., 1985]. In fact, cells lacking 6S RNA were found to grow indistinguishably from wild type under tested conditions, which unfortunately did not yield insight into 6S RNA function. Overexpression studies likewise did not uncover growth defects [Hsu et al., 1985]. Additional characterization of 6S RNA revealed that it existed in an RNA-protein complex, although the nature of the protein components were not readily identifiable at that time [Lee et al., 1978].