CAROTOVORA. SUBSP. CAROTOVORA.: CHARACTERIZATION OF RSMA, A RNA-BINDING PROTEIN WHICH MEDIATES POST-TRANSCRIPTIONAL REGULATION. ASITA MUKHERJEE, YANG LUI, YAYA CUI, C. KORSI DUMENYO AND ARUN K. CHATTERJEE Dept. of Plant Pathology, Univ of Missouri, Columbia, MO 65211 U. S.A. The enterobacterium, Erwinia carotovora subsp. carotovora (Ecc) causes tissue macerating (soft-rotting) disease in a wide variety of plants and plant organs. Like many other soft-rotting Erwinia spp., Ecc produces an assortment of degradadve es including pectate lyase isozymes (Pcls), polygalacmm (Peh), cellulase (Cel), proteaw (Prt), and pcctin lyasc (Pnl). Of these, Pels, Pnls and endo-Pchs are Imown to cause cell separation (i.e, tissue maceration) and changes in cell membrane permeability. Pel, Pch. Cel and Prt production is co-regulated at the level of transcription by the quorum sensing signal, N-acyl homoserine lactone (AHL) and an assortment of transcriptional factors [1]. We have documented that the production of these exoenzymes is also subject to post-transcriptional regulation by rsrnA, a regulator of secondary metabolism, which encodes a small basic RNA-binding protein RsmA affects other phenotypes as well, such as swarming motility, production of pigment, extracellular polysaccharide, AHL and glycogen and the elicitation of hypersensitive reaction (HR). rsmA-like genes occur in many, if not all, enterobacteria as well as other bacterial groups including several gram-positive bacteria [2]. RsmA, like CsrA, the E. coli homolog, binds RNA species, and indirect in vivo evidence suggests that this binding promotes message decay. In Ecc strain 71, two classes of PNA species have been detected: Class I RNAs which have been shown to be highly susceptible to RsmA action, i.e., pel-1, pch-1, hrpNEcc and ahII transcripts; Class II RNAS- which we not affected by RsmA, i.e.rsmB, rsmD and rpoS transcripts. The basis for these differential effects of RsmA is not yet known. Since RsmA plays a crucial role as a major post-transcriptional regulator, it is expected that its levels are tightly .regulated. Indeed, our. findings support this idea. For example, rmA expression in Ecc strain 71 occurs in a growth phase dependent manner and is controlled by a regulatory gene, rsmC, and the alternate sigma factor, RpoS. In RsmC. mutants, pel, pch, hrpNEcc and ahII transcripts are present at high levels, whereas the levels of rsmA. mRNA are much lower than in the RsmC+ bacteria. Multiple copies of rsmC+ DNA cause severe repression of pel, pch, hrpNEcc and ahII transcripts, but stimulate the levels of rsmA transcripts. Thus, rsmC could positively regulate rsmA expression. A similar, but quantitatively lower response is detected in RpoS deficient Ecc mutants and multiple copies of rpoS+ DNA, indicating that, like rsmC, it also positively regulates rsmA expression. This conclusion is strengthened by the characteristics of RsmC- and RpoS- double mutants, Studies have been initiated to elucidate the mechanisms by which RsmC and RpoS activate rsmA expression. References I. BanasF, van gijsegem F, Chatterjee AK. l994. Annu Rev. Phytopathol. 32, 201-234. 2. Cui Y, Chatterjee, Liu Y., Dumenyo CK, Chattejee AK. 1995. 1. Bacteriol. 177, 5108-5115.