Substrate Specificities and Expression Patterns Reflect the Evolutionary Divergence of Maltose ABC Transporters in Thermotoga maritima

Abstract

Duplication of transporter genes is apparent in the genome sequence of the hyperthermophilic bacterium Thermotoga maritima. The physiological impacts of these duplications are not well understood, so we used the bacterium's two putative maltose transporters to begin a study of the evolutionary relationship between a transporter's function and the control of expression of its genes. We show that the substrate binding proteins encoded by these operons, MalE1 and MalE2, have different substrate specificities and affinities and that they are expressed under different growth conditions. . Neither protein binds lactose. We examined the expression of these operons at both the transcriptional and translational levels and found that MalE1 is expressed in cells grown on lactose or guar gum and that MalE2 is highly expressed in starch-and trehalosegrown cells. Evidence is provided that malE1, malF1, and perhaps malG1 are cotranscribed and so constitute an operon. An open reading frame encoding a putative transcriptional regulatory protein adjacent to this operon (TM1200) is also up-regulated in response to growth on lactose. These evolutionarily related transporter operons have diverged both in function and expression to assume apparently different physiological roles.Annotation of the complete genome sequence of the hyperthermophilic, heterotrophic bacterium Thermotoga maritima indicates that 24% of the bacterium's open reading frames (ORFs) are most closely related to archaeal sequences (16). Subsequent studies have provided evidence that these genes were acquired by horizontal gene transfer (HGT). The closest known relative of T. maritima, Thermotoga species RQ2, acquired sugar ABC transporter and polysaccharide hydrolase genes independently of T. maritima, and this acquisition presumably provides selective nutritional advantages to this organism in its natural habitats (18). Intradomain gene acquisition or even gene duplication may confer novel selective advantages as well. This may be evident in the two putative maltose ABC transporters encoded in distant locations in the T. maritima genome (16). The amino acid sequences of the individual components of both transporters are very similar, suggesting a common evolutionary history (16). The transporter gene clusters (putative operons) may have arisen through operon duplication or horizontal acquisition of a second, orthologous operon by the ancestor of T. maritima. Evidence of a similar intradomain HGT of ABC transporter genes was found in two closely related archaea, Thermococcus litoralis and Pyrococcus furiosus (4). Regardless of their mechanism of duplication, the functions of these two T. maritima ABC transporters must confer distinct selective advantages since both have been retained.An organism will acquire a new catabolic trait after acquisition of new genes only if the genes are expressed and regulated in concert with other catabolism genes. Consequently, the evolution of the regions upstream of new genes and the binding of transcription factors to these regions...

Insights into thermoadaptation and the evolution of mesophily from the bacterial phylum Thermotogae

“…Carbohydrate utilization by T. maritima has been examined by studying the substrate specificities and affinities of its carbohydrate transporters (Nanavati et al 2005(Nanavati et al , 2006Cuneo et al 2009;Boucher and Noll 2011;Ghimire-Rijal et al 2014) and their transcriptional regulation in response to growth on different saccharides (Frock et al 2012). Information about substrate specificities, enzymatic activities, and catalytic mechanisms of many of T. maritima's glycoside hydrolases are also available (Kleine and Liebl 2006;Comfort et al 2007;Arti et al 2012), which has been used, for instance, to engineer an ␣-galactosidase from T. maritima into an efficient ␣-galactosynthase (Cobucci-Ponzano et al 2011).…”

An Expression-Driven Approach to the Prediction of Carbohydrate Transport and Utilization Regulons in theHyperthermophilic BacteriumThermotoga maritima

“…The complexity of its carbohydrate utilization strategies, revealed by genome sequencing (48) and through previous work (11,12,47,51), is surprising, given the primitive features of this microorganism. Considerable genomic plasticity has been observed even within the Thermotoga genus, with respect to the gene content of carbohydrate active enzymes and transporter subunits, which may to some extent relate to lateral gene transfer events (48,49).…”

Ligands of Thermophilic ABC Transporters Encoded in a Newly Sequenced Genomic Region of Thermotoga maritima MSB8 Screened by Differential Scanning Fluorimetry

“…2; also see Table S3 in the supplemental material). MalE2 binds maltose, maltotriose, and trehalose with respective K d values of 8.4 M, 11 M, and 9.5 M, but mannotetraose was not a ligand (13). At 10 mM sugar, we observed that maltose and maltotriose increased the thermal stability of MalE2 with ⌬T m values of 1.02°C and 6.85°C, respectively, while the ⌬T m value in the presence of 10 mM mannotetraose was 0.24°C ( Fig.…”

“…These proteins were previously characterized and their apparent dissociation constants (K d ) were determined using fluorescence spectroscopy (13,23). Fig.…”

“…All clones were validated by sequencing. The periplasmic substrate-binding protein genes malE1 and malE2 were cloned as previously described (13).…”