The lmaA gene of Listeria monocytogenes encodes a protein capable of inducing delayed-type hypersensitivity reactions in L. monocytogenes-immune mice (S. Göhmann, M. Leimeister-Wächter, E. Schiltz, W. Goebel, and T. Chakraborty, M. Microbiol. 4:1091-1099, 1990). Here we show that it is the last gene of the lma operon, which now comprises four genes, lmaDCBA. Maxicell analysis of peptides encoded by the lma operon identified four polypeptides of 16.7, 16.4, 14.9, and 21 kDa which correspond to the gene products encoded by the lmaD, -C, -B, and -A genes, respectively. Northern blot analysis of the lma operon showed that lmaA is expressed by two transcripts: the longer lmaDCBA transcript of 2,100 nucleotides, which was observed at growth temperatures of 37 and 20؇C, and a shorter transcript consisting of lmaBA, which is detected only at low temperatures (20؇C). Two promoters, one preceding the lmaD gene and another located upstream of the lmaB gene, were detected. An extended stem-loop structure resembling box elements found in other gram-positive pathogens was also present in the lmaC-lmaB intergenic region. By immunoblot analysis, we found that although LmaA was produced at both temperatures (20 and 37؇C), it was secreted into culture supernatants only at 20؇C. However, LmaA lacks a bona fide signal peptide sequence and could, like flagellin, be secreted by a type III transport system. DNA hybridization studies indicate that the lma operon is species specific and restricted to pathogenic strains of L. monocytogenes.The gram-positive, facultative intracellular bacterium Listeria monocytogenes is a foodborne pathogen responsible for infections in immunocompromised persons and pregnant women. Listerial infections can result in meningitis, meningoencephalitis, septicemia, and death (10, 12, 16). The consumption of contaminated food has been shown to be the major route of infection. In addition to causing sporadic listeriosis, L. monocytogenes has been implicated in several epidemics in Europe and North America (2, 37). The ability of L. monocytogenes to grow at 4ЊC, a temperature at which many off-theshelf foodstuffs are stored, opens up a unique environment for listerial enrichment (16,43). Listerial products that are expressed at low growth temperatures are therefore a relevant area of study.Virulence factors necessary for entry, intracellular survival, multiplication, and motility have been previously characterized. The genes encoding these factors are organized in two clusters on the listerial chromosome. The main cluster contains the genes coding for hemolysin (hly), phospholipases (plcA, plcB), metalloprotease (mpl), and actin nucleation factor (actA) and includes the virulence regulator protein PrfA (6, 18-21, 27, 28). The internalin genes inlA and inlB, which are required for uptake of the bacteria into the eukaryotic cell, are arranged as an operon close to but separate from the other virulence locus (29). The expression of the inlA and inlB genes is regulated by both PrfA-dependent and -independent mechani...
