DIFFERENTIAL EXPRESSION OF VIRULENCE GENES AND MOTILITY IN RALSTONIA (PSEUDOMONAS) SOLANACEARUM DURING EXPONENTIAL-GROWTH

A complex network regulates virulence in Ralstonia solanacearum (forme rly Pseudomonas solanacearum); central to this system is PhcA, a LysR- type transcriptional regulator, We report here that two PhcA-regulated virulence factors, endoglucanase (Egl) and acidic exopolysaccharide I (EPS I), and motility are expressed differentially during exponential growth in batch cultures, Tests with strains carrying lacZ fusions in a wild-type genetic background revealed that expression (on a per-cel l basis) of phcA was constant but expression of egl and epsB increased 20- to 50-fold during multiplication from 1 x 10(7) to 5 x 10(8) CFU/ ml, Expression of xpsR, an intermediate regulator downstream of PhcA i n the regulatory cascade for eps expression, was similar to that of ep sB and egl, Motility track photography revealed that all strains were essentially nonmotile at 10(6) CFU/ml, As cell density increased, 30 t o 50% of wild-type cells were motile between 10(7) and 10(8) CFU/ml, b ut this population was again nonmotile at 10(9) CFU/ml, In contrast, a bout 60% of the cells of phcB and phcA mutants remained motile at 10(9 ) CFU/ml. Expression of phcB which is not positively regulated by PhcA , was the inverse of epsB, egl, and xpsR (i.e., it decreased 20-fold a t high cell density). PhcB is essential for production of an extracell ular factor, tentatively identified as 3-hydroxypalmitic acid methyl e ster (3-OH PAME), that might act as an exponential-phase signal to act ivate motility or expression of virulence genes, However, growth of th e lacZ fusion strains in medium containing excess 3-OH PAME did not re sult in motility or expression of virulence genes at dramatically lowe r cell densities, suggesting that 3-OH PAME is not the only factor con trolling these traits.