SummaryThe maltose regulon consists of 10 genes encoding a multicomponent and binding protein-dependent ABC transporter for maltose and maltodextrins as well as enzymes necessary for the degradation of these sugars. MalT, the transcriptional activator of the system, is necessary for the transcription of all mal genes. MalK, the energy-transducing subunit of the transport system, acts phenotypically as repressor, particularly when overproduced. We isolated an insertion mutation that strongly reduced the repressing effect of overproduced MalK. The affected gene was sequenced and identified as mlc, a known gene encoding a protein of unknown function with homology to the Escherichia coli NagC protein. The loss of Mlc function led to a threefold increase in malT expression, and the presence of mlc on a multicopy plasmid reduced malT expression. By DNaseI protection assay, we found that Mlc protected a DNA region comprising positions þ 1 to þ 23 of the malT transcriptional start point. Using a mlc -lacZ fusion in a mlc and mlc þ background, we found that Mlc represses its own expression. As Mlc also regulates another operon (manXYZ, see pages 369-379 of this issue), it may very well constitute a new global regulator of carbohydrate utilization.

The maltose system in Escherichia coli consists of cell envelope-associated proteins and enzymes that catalyze the uptake and utilization of maltose and a,1-4-linked maltodextrins. The presence of these sugars in the growth medium induces the maltose system (exogenous induction), even though only maltotriose has been identified in vitro as an inducer (0. Raibaud and E. Richet, J. Bacteriol., 169:3059-3061, 1987). Induction is dependent on MalT, the positive regulator protein of the system. In the presence of exogenous glucose, the maltose system is normally repressed because of catabolite repression and inducer exclusion brought about by the phosphotransferase-mediated vectorial phosphorylation of glucose. In contrast, the increase of free, unphosphorylated glucose in the cell induces the maltose system. A ptsG ptsM glk mutant which cannot grow on glucose can accumulate ['4CJglucose via galactose permeases. In this strain, internal glucose is polymerized to maltose, maltotriose, and maltodextrins in which only the reducing glucose residue is labeled. This polymerization does not require maltose enzymes, since it still occurs in malT mutants. Formation of maltodextrins from external glucose as well as induction of the maltose system is absent in a mutant lacking phosphoglucomutase, and induction by external glucose could be regained by the addition of glucose-lphosphate entering the cells via a constitutive glucose phosphate transport system. malQ mutants, which lack amylomaltase, are constitutive for the expression of the maltose genes. This constitutive nature is due to the formation of maltose and maltodextrins from the degradation of glycogen.The Escherichia coli maltose system consists of a maltodextrin-specific pore (encoded by lamB) (17, 30) in the outer membrane and a binding-protein-dependent transport system in the cell envelope (encoded by malE malF malG malK) (38), as well as one periplasmic enzyme (encoded by malS) (35) and three cytoplasmic enzymes (encoded by malQ, malP, and malZ) (27,34,41). Expression of all mal genes depends on the positive regulator MalT (33).The transport system (11,12,20,24,40) can recognize and accumulate maltose and linear a,1-4-linked maltodextrins up to a chain length of seven glucose units (15). The major enzymes of the system (see Fig. 1) are the cytoplasmic amylomaltase (MalQ) (42) and maltodextrin phosphorylase (MalP) (34). Amylomaltase recognizes maltotriose and larger maltodextrins (donors), cleaving off the reducing glucose residue and transferring the remaining dextrinyl residue onto the nonreducing end of maltodextrin (acceptors), including maltose and glucose. With maltotriose, the smallest donor substrate, as well as with longer linear maltodextrins, amylomaltase thus produces glucose and longer maltodextrins (26). Maltodextrin phosphorylase subsequently releases glucose-i-phosphate from the nonreducing end of maltodextrins with a minimal chain length of five glucose residues (37). The glucose and glucose-l-phosphate are both transformed into glucose-6-phosphat...

The nucleotide sequence of the region between the oad gene, encoding the host specificity protein, and the right-terminal repetition of bacteriophage T5 DNA was determined. Five small open reading frames, the first of which was called llp, were detected, which apparently formed an operon transcribed from a promoter that overlapped the oad promoter. Both promoters were confirmed by primer extension assays. Using mRNA isolated at different times after T5 infection, the llp and oad promoters were identified as early and late promoters, respectively. The N-terminus of the llp gene product possess a signal sequence and a processing site characteristic of lipoproteins. After subcloning and expression of llp, its product Llp was identified as a 7.8 kDa polypeptide. Acylation of Llp was confirmed by addition of globomycin, which resulted in the accumulation of the unprocessed precursor form. FhuA+ cells synthesizing Llp were resistant to phage T5. Resistance was caused by inhibition of adsorption of T5 to its FhuA receptor protein. Resistance could be overcome by derepression of fhuA transcription, suggesting a blocking of FhuA by direct interaction with Llp. Since Llp-mediated T5 resistance has several aspects in common with the phenomenon of lysogenic conversion, we suggest that it should be called lytic conversion.

The maltose regulon consists of 10 genes encoding an ABC transporter for maltose and maltodextrins as well as enzymes necessary for their degradation. MalK, the energy‐transducing subunit of the transport system, acts phenotypically as a repressor of MalT, the transcriptional activator of the mal genes. Using MacConkey maltose indicator plates we isolated an insertion mutation that strongly reduced the repressing effect of overproduced MalK. The insertion had occurred in treR encoding the repressor of the trehalose system. The loss of TreR function led to derepression of treB encoding an enzymeIITre of the PTS for trehalose and of treC encoding TreC, the cytoplasmic trehalose‐6‐phosphate hydrolase. Further analysis revealed that maltose can enter the cell by facilitated diffusion through enzymeIITre, thus causing induction of the maltose system. In addition, derepression of TreC by itself caused induction of the maltose system, and a mutant lacking TreC was reduced in the uninduced level of mal gene expression indicating synthesis of endogenous inducer by TreC. Extracts containing TreC transformed [14C]‐maltose into another 14C‐labelled compound (preliminarily identified as maltose 1‐phosphate) that is likely to be an alternative inducer of the maltose system.

No abstract

No abstract