A type II protein secretory pathway required for levansucrase secretion by Gluconacetobacter diazotrophicus.
The endophytic diazotroph Gluconacetobacter diazotrophicus secretes a constitutively expressed levansucrase (LsdA, EC 2.4.1.10) to utilize plant sucrose. LsdA, unlike other extracellular levansucrases from gram-negative bacteria, is transported to the periplasm by a signal-peptide-dependent pathway. We identified an unusually organized gene cluster encoding at least the components LsdG, -O, -E, -F, -H, -I, -J, -L, -M, -N, and -D of a type II secretory system required for LsdA translocation across the outer membrane. Another open reading frame, designated lsdX, is located between the operon promoter and lsdG, but it was not identified in BLASTX searches of the DDBJ/EMBL/GenBank databases. The lsdX, -G, and -O genes were isolated from a cosmid library of strain SRT4 by complementation of an ethyl methanesulfonate mutant unable to transport LsdA across the outer membrane. The downstream genes lsdE, -F, -H, -I, -J, -L, -M, -N, and -D were isolated through chromosomal walking. The high G+C content (64 to 74%) and the codon usage of the genes identified are consistent with the G+C content and codon usage of the standard G. diazotrophicus structural gene. Sequence analysis of the gene cluster indicated that a polycistronic transcript is synthesized. Targeted disruption of lsdG, lsdO, or lsdF blocked LsdA secretion, and the bacterium failed to grow on sucrose. Replacement of Cys(162) by Gly at the C terminus of the pseudopilin LsdG abolished the protein functionality, suggesting that there is a relationship with type IV pilins. Restriction fragment length polymorphism analysis revealed conservation of the type II secretion operon downstream of the levansucrase-levanase (lsdA-lsdB) locus in 14 G. diazotrophicus strains representing 11 genotypes recovered from four different host plants in diverse geographical regions. To our knowledge, this is the first report of a type II pathway for protein secretion in the Acetobacteraceae.
Arrieta
Juan G
JG
Plant-Microbe Interactions Lab, Plant Division, Center for Genetic Engineering and Biotechnology, P.O. Box 6162, Havana 10600, Cuba. arrieta@cigb.edu.cu
Sotolongo
Mailin
M
Menéndez
Carmen
C
Alfonso
Dubiel
D
Trujillo
Luis E
LE
Soto
Melvis
M
Ramírez
Ricardo
R
Hernández
Lázaro
L
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D015964:Gene Expression Regulation, Bacterial; D041821:Gluconacetobacter / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D006026:Glycoside Hydrolases / Q000235:genetics / Q000378:metabolism; D006602:Hexosyltransferases / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D005810:Multigene Family; D016297:Mutagenesis, Site-Directed; D012150:Polymorphism, Restriction Fragment Length; D013395:Sucrose / Q000378:metabolism
D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D013395:Sucrose; D006602:Hexosyltransferases; C021513:levansucrase; D006026:Glycoside Hydrolases; C021512:levanase