1. The reason why you don't have the ABC transporter orthology for maltose and sn-Glycerol-3-phosphate is probably due to lack the conventional mechanisms for ATP generation, such as glycolysis or arginine breakdown, present in other mycoplasmas.
2. REP( repetitive extragenic palindrome) sequence element located downstream of the genes for the binding proteins in E.coli . And this element may serve a similar stabilizing function for the 5’ end of the mRNA there.
3. The maltose transport complex (MTC) is a member of the ATP-binding cassette superfamily of membrane transport proteins and is a model for understanding the folding and assembly of hetero-oligomeric membrane protein complexes. The MTC is made up of two integral membrane proteins, MalF and MalG, and a peripheral membrane protein, MalK. MalK forms a dimer independent of its assembly into the MalFGK2 ATP-binding cassette transporter of Escherichia coli . (J Biol Chem 1999 Mar 5;274(10):6259-64 Kennedy KA, Traxler B PMID: 10037713, UI: 99156903).
Interactions between the nucleotide-binding protein, MalK, and the transmembrane proteins, MalF and MalG has been studied. These experimental results suggest that the putative helical domain of the nucleotide-binding domains is involved, through its conformational changes, in the coupling between the transmembrane domains and ATP binding/hydrolysis at the nucleotide-binding domains. (Mol Microbiol 1998 Oct;30(2):353-63 In vitro interaction between components of the inner membrane complex of the maltose ABC transporter of Escherichia coli: modulation by ATP.Mourez M, Jehanno M, Schneider E, Dassa E PMID: 9791180, UI: 99009364).
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. Experimental 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. (Mol Microbiol 1999 May;32(4):777-88 The role of the trehalose system in regulating the maltose regulon of escherichia coli. Decker K, Gerhardt F, Boos W PMID: 10361281, UI: 99291045).
Transcription of the mal regulon of Escherichia coli K-12 is regulated by the positive activator, MalT. In the presence of ATP and maltotriose, MalT binds to decanucleotide MalT boxes that are found upstream of mal promoters and activates transcription at these sites. it was found that overexpression of the MalK protein resulted in very low levels of mal gene transcription. The experimental results are consistent with a model in which the inactive form of MalK down-modulates MalT and decreases transcription, whereas the active form of MalK does not. This model suggests that bacteria may be able to couple information about extracellular substrate availability to the transcriptional apparatus via the levels of ATP hydrolysis associated with transport. ( Mol Microbiol 1998 Nov;30(3):535-46 The ATP-binding cassette subunit of the maltose transporter MalK antagonizes MalT, the activator of the Escherichia coli mal regulon. Panagiotidis CH, Boos W, Shuman HA PMID: 9822819, UI: 99042180).
The maltose system of Escherichia coli offers an unusually rich set of enzymes, transporters, and regulators as objects of study. This system is responsible for the uptake and metabolism of glucose polymers (maltodextrins), which must be a preferred class of nutrients for E. coli in both mammalian hosts and in the environment. Because the metabolism of glucose polymers must be coordinated with both the anabolic and catabolic uses of glucose and glycogen, an intricate set of regulatory mechanisms controls the expression of mal genes, the activity of the maltose transporter, and the activities of the maltose/maltodextrin catabolic enzymes. The ease of isolating many of the mal gene products has contributed greatly to the understanding of the structures and functions of several classes of proteins. Not only was the outer membrane maltoporin, LamB, or the phage lambda receptor, the first virus receptor to be isolated, but also its three-dimensional structure, together with extensive knowledge of functional sites for ligand binding as well as for phage lambda binding, has led to a relatively complete description of this sugar-specific aqueous channel. The periplasmic maltose binding protein (MBP) has been studied with respect to its role in both maltose transport and maltose taxis. Again, the combination of structural and functional information has led to a significant understanding of how this soluble receptor participates in signaling the presence of sugar to the chemosensory apparatus as well as how it participates in sugar transport. The maltose transporter belongs to the ATP binding cassette family, and although its structure is not yet known at atomic resolution, there is some insight into the structures of several functional sites, including those that are involved in interactions with MBP and recognition of substrates and ATP. A particularly astonishing discovery is the direct participation of the transporter in transcriptional control of the mal regulon. The MalT protein activates transcription at all mal promoters. A subset also requires the cyclic AMP receptor protein for transcription. The MalT protein requires maltotriose and ATP as ligands for binding to a dodecanucleotide MalT box that appears in multiple copies upstream of all mal promoters. Recent data indicate that the ATP binding cassette transporter subunit MalK can directly inhibit MalT when the transporter is inactive due to the absence of substrate. Despite this wealth of knowledge, there are still basic issues that require clarification concerning the mechanism of MalT-mediated activation, repression by the transporter, biosynthesis and assembly of the outer membrane and inner membrane transporter proteins, and interrelationships between the mal enzymes and those of glucose and glycogen metabolism.( Microbiol Mol Biol Rev 1998 Mar;62(1):204-29 Maltose/maltodextrin system of Escherichia coli : transport, metabolism, and regulation. Boos W, Shuman H PMID: 9529892, UI: 98190797).