Snellings N J, Johnson E M, Kopecko D J, Collins H H, Baron L S
J Bacteriol. 1981 Feb;145(2):1010-7. doi: 10.1128/jb.145.2.1010-1017.1981.
Certain strains of the genus Citrobacter exhibit a variable expression of the Vi surface antigen that appears to involve a special mechanism for regulation of gene expression. Two nonlinked chromosomal loci, viaA and viaB, are known to determine nonvariable Vi antigen expression in strains of Salmonella. To confirm the presence of analogous loci in Citrobacter and to ascertain whether either of them is involved in variable Vi antigen expression in this organism, donor strains were constructed from Citrobacter freundii WR7004 and used to transfer their Vi antigen-determining genes to ViaA- and ViaB- Salmonella typhi recipient strains. Vi antigen expression in C. freundii was found to be controlled by loci analogous to the Salmonella via genes. S. typhi recipients of the C. freundii viaA+ genes were restored to the full, continuous expression of the Vi antigen normally seen in S. typhi. Thus, the C. freundii viaA genes appeared to play no role in the variable expression of the Vi antigen. In contrast, S. typhi recipients of the C. freundii viaB+ genes exhibited the rapid, reversible alternation between full Vi antigen expression and markedly reduced Vi antigen expression that was seen to occur in the C. freundii parent. The C. freundii viaB locus was thus identified as the one whose genes are regulated so as to produce variable Vi antigen expression. Genes determining another C. freundii surface antigen, the synthesis of which is not affected by the mechanism regulating Vi expression, were coinherited with the C. freundii viaB+ genes. An invertible, insertion sequence element located within the C. freundii viaB locus is proposed to account for the regulation of variable Vi antigen expression.
柠檬酸杆菌属的某些菌株表现出Vi表面抗原的可变表达，这似乎涉及一种特殊的基因表达调控机制。已知两个不连锁的染色体位点viaA和viaB可决定沙门氏菌菌株中Vi抗原的非可变表达。为了确认柠檬酸杆菌中是否存在类似的位点，并确定其中任何一个是否参与该生物体中Vi抗原的可变表达，构建了来自弗氏柠檬酸杆菌WR7004的供体菌株，并用于将其Vi抗原决定基因转移到ViaA - 和ViaB - 的伤寒沙门氏菌受体菌株中。发现弗氏柠檬酸杆菌中Vi抗原的表达受与沙门氏菌via基因类似的位点控制。弗氏柠檬酸杆菌viaA + 基因的伤寒沙门氏菌受体恢复到通常在伤寒沙门氏菌中看到的Vi抗原的完全、持续表达。因此，弗氏柠檬酸杆菌viaA基因似乎在Vi抗原的可变表达中不起作用。相反，弗氏柠檬酸杆菌viaB + 基因的伤寒沙门氏菌受体表现出在完全Vi抗原表达和明显降低的Vi抗原表达之间快速、可逆的交替，这在弗氏柠檬酸杆菌亲本中也能看到。因此，弗氏柠檬酸杆菌viaB位点被确定为其基因受到调控从而产生可变Vi抗原表达的位点。决定弗氏柠檬酸杆菌另一种表面抗原合成的基因，其合成不受调节Vi表达的机制影响，与弗氏柠檬酸杆菌viaB + 基因共同遗传。位于弗氏柠檬酸杆菌viaB位点内的一个可逆插入序列元件被认为是可变Vi抗原表达调控的原因。
