INTERNALIN-MEDIATED INVASION OF EPITHELIAL-CELLS BY LISTERIA-MONOCYTOGENES IS REGULATED BY THE BACTERIAL-GROWTH STATE, TEMPERATURE AND THE PLEIOTROPIC ACTIVATOR PRFA

Entry of Listeria monocytogenes into epithelial cells requires express ion of inIA, the first gene of an operon comprising two genes: inIA, w hich encodes internalin, a 800-amino-acid protein, and inIB, which enc odes a 630-amino-acid protein. We report here that the inI locus is tr anscribed on two transcripts in constant relative ratio: a 5 kb transc ript spanning inIA and inIB, and a 2.9 kb transcript that covers only inIA. The promoter is located 397 bp from the GTG initiator of inIA an d displays in its -35 region a palindrome similar to that found in pro moters controlled by the pleiotropic activator prfA. Transcription of the inI locus is, as are several other L. monocytogenes virulence gene s, activated by prfA and regulated by temperature- with higher express ion at 37-degrees-C versus 25-degrees-C-and bacterial growth state. It is maximal during exponential growth and correlates with maximal inva sivity of the bacteria in the human epithelial cell line Caco-2. It al so correlates with maximum amounts of internalin present on the bacter ial surface. Internalin is also detected in substantial amounts in cul ture supernatants. Taken together, these data suggest that surface-bou nd internalin plays an important role in bacterial entry but do not ex clude a role for the released form.