Analysis of Mesorhizobium loti glycogen operon: effect of phosphoglucomutase (pgm) and glycogen synthase (g/gA) null mutants on nodulation of Lotus tenuis

LEPEK VC,; D'ANTUONO AL; TOMATIS PE; UGALDE JE; GIAMBIAGI S; UGALDE RA

MOLECULAR PLANT-MICROBE INTERACTIONS

Año: 2002 vol. 15 p. 368 - 375

0894-0282

   The phosphoglucomutase (pgm) gene codes for a key en- zyme required for the formation of UDP-glucose and ADP- glucose, the sugar donors for the biosynthesis of glucose containing polysaccharides. A Mesorhizobium loti pgm null mutant obtained in this study contains an altered form of lipopolysaccharide (LPS), lacks exopolysaccharide (EPS), ? cyclic glucan, and glycogen and is unable to nodulate Lotus tenuis. The nonnodulating phenotype of the pgm mutant was not due to the absence of glycogen, since a glycogen synthase (glgA) null mutant effectively nodulates this leg- ume. In M. loti, pgm is part of the glycogen metabolism gene cluster formed by  GlgP  (glycogen phosphorylase), glgB (glycogen branching), glgC (ADP-glucose pyrophos- phorylase), glgA, pgm, and glgX (glycogen debranching). The genes are transcribed as a single transcript from glgP to at least pgm under the control of a strong promoter (pro- moter I) upstream of glgP. An alternative promoter (pro- moter II), mapping in a 154-bp DNA fragment spanning 85 bp upstream of the glgA start codon and the first 69 bp of the glgA coding region, controls the expression of glgA and pgm, independently of the rest of the upstream genes. Primer extension experiments showed that transcription starts 19 bp upstream of the glgA start codon.