Background

Motivation

Water quality is of growing concern in Ghana with about 60% of the water bodies being polluted and most are in critical condition (Ampomah, 2017). The main cause of this pollution is unregulated industrial activities such as illegal mining which leaches dangerous heavy metals into the water bodies. Many of these river bodies have for many years served as a source of water for residents and have now become too dangerous to use. Notable among them are River Pra, Daboase, and River Birim in Ghana. These rivers and their tributaries have been reported to contain levels of heavy metals such as cadmium, mercury and lead much higher than the maximum accepted concentrations established by the World Health Organisation (WHO). In the Birim river, they have found water samples which have shown lead, mercury, and cadmium levels 400, 180, and 15 times over the WHO limit, respectively (Afum and Owusu, 2016).

Heavy metal pollution poses a serious public health risk, threatening the health of communities dependent on these water bodies. Ingestion of heavy metal-contaminated water can lead to kidney damage and neurotoxic effects (mainly affecting children). There is also a growing concern about heavy metal accumulation in soil and crops since most of these illegal mining sites are located near farmlands. An assessment of heavy metal contamination in surface soils and plants on the West Coast of Ghana found lead was observed in the highest concentrations in plant samples, exceeding by over 17-fold, the Food and Agricultural Organization (FAO) permissible limit of 5mg/kg. Cadmium (17.47 mg/kg) was also detected in plants at levels that surpassed the 0.20mg/kg limit set by the FAO. Given the prevalence of these metals at alarming concentrations, this is undoubtedly a threat to plant productivity, thus a threat to food security and public health (Fosu-Mensah et al. 2018).

Nature has already certain examples whereby certain organisms tolerate high concentrations of heavy metals. These mechanisms of heavy metal tolerance can be explored as potential solutions for heavy metal bioremediation.

Metallothioneins

Based on a preliminary literature review, we identified these metal-chelating proteins, metallothioneins (MTs) responsible for the heavy metal tolerance in several organisms. First identified in blue mussels (Mytilus edulis), MTs are intracellular, cysteine-rich proteins, instrumental in metal chelation (Coyle et al. 2002). Comprising metal binding motifs which utilise thiol groups on cysteine sidechains, MTs can form coordinate covalent bonds with various metals, effectively sequestering them from the surrounding environment.

Taking advantage of this inherent metal-binding capability, we wanted to carry out directed evolution of MTs to improve metal binding capacity through error-prone PCR-mediated random mutagenesis.