Description of a new potential aggregation factor from the Streptococcus thermophilus genome

Abstract

Autoaggregation, the ability to self-aggregate, is widespread among both Gram-positive and Gram-negative bacteria. The functional role of aggregation is not fully understood, but it is believed to be involved in the adaptation of bacteria to environmental conditions (PMID: 31294207). One interesting class of compounds responsible for the aggregation of lactic acid bacteria is aggregation factors—surface high-molecular-weight proteins rich in threonine and lysine (PMID: 30027759). Recently, our research group discovered a new strain of Streptococcus thermophilus in the mountainous regions of Serbia, exhibiting an aggregation phenotype. Aggregation phenotype was confirmed visually and using microscopy. Complete genome of Agg+ strain was sequenced using NGS and a gene encoding a potential aggregation factor, which was named aggS was identified. The predicted threonine (12.5%) and lysine (10.5%) rich protein contains 2367 amino acids, with an average molecular weight of 255986.63 Da. AggS also contains two cysteine residues, whereas previously well-described aggregation factors of this type did not contain any cysteine residues. The predicted protein includes an N-terminal YSIRK-like signal sequence and an LPXTG cell wall anchor domain. It has 6 Mucin binding domain repeats alternating with 6 Mub B2-like domain repeats. Additionally, we found a region resembling an ice-binding domain. Given that these bacteria endure prolonged periods of low temperatures, it can be speculated that this surface membrane protein also helps the bacteria withstand freezing. The fact that the alignment using BLASTp revealed AggS to be most closely related to an uncharacterised protein from the genome of Lactococcus garvieae, along with the discovery of a transposase gene sequence upstream of the gene, suggests that the aggregation factor was likely acquired through horizontal gene transfer. We plan to clone it into a shuttle vector and investigate the aggregation phenotype using a heterologous expression system in Lactococcus lactis, as well as explore its other functions.